Short answer is that the spell targets the creature in its current form.
Who Is Really In There?
Regardless of the form the creature is in after the first True Polymorph, or was originally in, the creature gets a saving throw if it is unwilling to change under a given instance of this spell.
True Polymorph Targets a single Creature
True Polymorph creature-to-creature version results in a creature with a new form: another creature. Any subsequent casting is an attempt to overwrite a previous casting, but that does not change the fact that the second instance is being cast upon a creature (the new form): a creature affected by the spell gets changed into (some new form) under two conditions:
- Willingly
- On a failed Wisdom save.
Choose one creature or non-magical object that you can see within range. You transform the creature into a different creature, (SRD Spell Description, p. 185).
The second wizard has only one creature to changed into that owlbear, not two. If the first caster loses concentration, the dragon becomes a fighter. That is because the two don't happen at once, as you described in your example.
This is a turn based game, so something has to happen first. If the second wizard cast the spell while the first has concentration on the Polymorph, that gets adjudicated first. If the first wizard loses concentration before the second wizard casts the spell, that gets revolved (dragon becomes fighter) first.
This means that if the first wizard is still concentrating, the only creature the second wizard has to work with is the dragon.
If the second wizard casts the spell on the dragon while the other wizard is concentrating, and not after it is permanent, then resolve the conflict with the following in mind:
An unwilling creature can make a Wisdom saving throw, and if it succeeds, it isn’t affected by this spell.
1) Have the dragon make a saving throw. It is the creature being targeted. If he willingly accepts True Polymorph from the first wizard, this save should probably be rolled first: he's the creature the other wizard is trying to change into an owlbear. If he is willing to be changed, no saving throw is needed. We then address the problem of two instances of the spell trying to affect the same creature at the same time.
The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
If the dragon fails the save -- true polymorph uses the new form's stats so our changed Fighter must use the Wisdom Save for the dragon ---- then it gets turned into an owlbear ... pending the outcome of the contest between two wizards.
If the Fighter/Dragon makes the save, then no change happens.
(You could do this contest first, and if the second wizard wins, then roll the save).
Credit to @Dale M for raising this point: when two of the same spell are applied to the same target, only one affects it. (PHB p. 205)
* In the case of True Polymorph versus Polymorph, 9th level spell beats
4th level spell
* In the case of Shapechange versus True Polymorph, it is 9th level
versus 9th level spell, but since the Shapechange is cast on "self"
you are dealing with the save versus True Polymorph in the
Shapechanged form
How do you decide between the first and second instance of the True Polymorph spell?
The first could be overcome by the second wizard's casting based on a different game rule, since there can be only one instance of that spell affecting that creature. Two characters are trying to do the same thing at once? Use rules for a Contest.
2) Who wins the Contest?(p. 58 of the Basic Rules).
Contests
Sometimes one character’s or monster’s efforts are directly opposed to
another’s. This can occur when both of them are trying to do the same
thing and only one can succeed ... In situations like these, the
outcome is determined by a special form of ability check, called a
contest. Both participants in a contest make ability checks
appropriate to their efforts. They apply all appropriate bonuses and
penalties, but instead of comparing the total to a DC, they compare
the totals of their two checks. The participant with the higher check
total wins the contest. That character or monster either succeeds at
the action or prevents the other one from succeeding.
I'd recommend using the Intelligence (spell casting stat) as the basis for the contest. That means that the situation you set up about dueling concentration doesn't have a chance to happen -- it's a turn based game.
Result of the contest is either:
- The first keeps his concentration up, and he wins
- The second overcomes him as a result of the Contest being resolved
and the dragon is changed into an owlbear.
- (Apply saving throw, or not, based on the target being willing, or not).
Do the rules specifically state that for this spell? No. What the Contest rule provides is a simple way to resolve it, keeping in mind that things happen in a sequence in a turn based game, and that only one instance of this spell can apply to a given creature at a time.
Further comments
Note the position the second wizard is in, in terms of sequence of events.
True Polymorph / Casting Time: 1 action (SRD. p. 185)
Either the second wizard succeeds in his attempt to change the dragon into something else, or he fails. You can do it by save, by contest, or both if the dragon/fighter is not willing to change into a new form.
If the fighter was not willingly changed into a dragon, he might accept the second wizard's spell and not fight it. Only an unwilling creature rolls a save versus this spell.
An unwilling creature can make a Wisdom saving throw, and if it succeeds, it isn’t affected by this spell.
How does the spell work? First, it changes a creature's shape. Then, the wizard concentrate on the creature staying in the new shape, either short term or long term. The second wizard cannot concentrate on maintaining the new shape of a creature that he did not yet change into an owlbear.
This step by step approach avoids having to rule about "who is really in there?". Even though the polymorphed creature retains its personality and alignment (who it is), you can make a ruling on whether the warrior rolls a save versus the second spell, or not. Just ask the player.
Each casting of the spell allows a save for an unwilling creature who is the target.
This interaction goes both ways: if the second wizard loses concentration, if he targeted the fighter, then the owlbear becomes a fighter. If he targeted the dragon, then the owlbear becomes a dragon.
The owlbear only changes into a dragon if the dragon was permanent. Otherwise, he becomes the Fighter, since the contest had to be decided in the second wizard's favor in order to turn the dragon into the owlbear in the first place.
The spell doesn't prohibit the gaining of experience, so it doesn't… at least not automatically.
You bring up some examples where it might make sense to not gain XP though: the sub-human intelligence giant bug and the alien-minded elemental. A DM might look at those and decide that the former is incapable of learning, and the latter is capable of learning but their normal mind would be incapable of applying those learning experiences later to their true form's life.
The other two examples (the minotaur and the lamia) might inspire the thought that experience gained is still useful, but not fully applicable to their true form. A DM might think that maybe they should earn XP, but not the full amount…
And maybe a DM might think that gaining XP while effectively being a member of a different class should… have an effect? Maybe it can be handled by multiclassing once returned to true form, or just wiped out, or transfered at a reduced rate…?
A DM could have all kinds of useful thoughts about these situations, which the book doesn't answer for them.
Fortunately this doesn't have to be complicated, because it's a rule that the DM can make such rulings according to what makes sense and is best for their group's game. It's their job within the game to make decisions based on real game contexts that the rules don't or can't foresee. The DM at the table is far better equipped to make choices that are best for their game than a designer far away and years in the past, anyway.
So if you're the DM: decide. If you're not the DM: ask your DM.
Best Answer
No, because you don't have any aboleth ancestors.
An aboleth has a flawless memory of the memories of all of its aboleth ancestors, and you do gain this ability when you turn yourself into one. However, if you use a spell to turn yourself into an aboleth, you don't have any aboleth ancestors; you're forming an entirely new line of aboleths. If you had any aboleth children, they'd inherit your memories, but as the spellcaster, you don't have any ancestors to inherit the memories of.