As Flenyar quoted in his answer:

Each time he gains a new level, he chooses two classes, takes the best aspects of each and applies them to his characteristic.

This implies that if one class would gain a BAB at it's respective level, you do as well. Note that unlike normal multiclass characters, your stats do depend on what order you take the classes in.

Notice how Flenyar's progression (6 levels of Fighter/Rogue, then 4 levels of Wizard/Rogue) gives a BAB of +9, but the following progression gives you a full BAB of +10, even though the character is still a Fighter 6/Wizard 4//Rogue 10:

1. Fighter 1 / Rogue 1 (+1 [Fighters gain +1 every level])
2. Fighter 2 / Rogue 2 (+1 [Fighters gain +1 every level])
3. Fighter 3 / Rogue 3 (+1 [Fighters gain +1 every level])
4. Fighter 4 / Rogue 4 (+1 [Fighters gain +1 every level])
5. Fighter 5 / Rogue 5 (+1 [Fighters gain +1 every level])
6. Wizard 1 / Rogue 6 (+1 [Rogues gain +1 at 6th level])
7. Wizard 2 / Rogue 7 (+1 [Both Wizards and Rogues BAB improves])
8. Wizard 3 / Rogue 8 (+1 [Rogues gain +1 at 8th level])
9. Fighter 6 / Rogue 9 (+1 [Fighters gain +1 every level])
10. Wizard 4 / Rogue 10 (+1 [Both Wizards and Rogues BAB improves])

Total BAB at level 10 is +10.

This means that even a Wizard//Sorcerer gestalt (or any other two low BAB classes) can have a perfect BAB if you take just one level of another perfect BAB class:

1. Wizard 1 / Fighter 1 (+1 [Fighters gain +1 every level])
2. Wizard 2 / Sorcerer 1 (+1 [Wizards gain +1 at 2nd level])
3. Wizard 3 / Sorcerer 2 (+1 [Sorcerer gain +1 at 2nd level])
4. Wizard 4 / Sorcerer 3 (+1 [Wizards gain +1 at 4th level])
5. Wizard 5 / Sorcerer 4 (+1 [Sorcerer gain +1 at 4th level])
6. Wizard 6 / Sorcerer 5 (+1 [Wizards gain +1 at 6th level])
7. Wizard 7 / Sorcerer 6 (+1 [Sorcerer gain +1 at 6th level])
8. Wizard 8 / Sorcerer 7 (+1 [Wizards gain +1 at 8th level])
9. Wizard 9 / Sorcerer 8 (+1 [Sorcerer gain +1 at 8th level])
10. Wizard 10 / Sorcerer 9 (+1 [Wizards gain +1 at 10th level])

Total BAB at level 10 for this Wizard 10//Fighter 1/Sorcerer 9 is +10 (perfect.) This cannot be determined without the order of the levels taken.

However...

...the DM may overrule this, based on the wording of this rule:

Base Attack Bonus: Choose the better progression from the two classes. (Emphasis mine.)

It is conceivable, then, that the Wizard 10//Fighter 1/Sorcerer 9 is a character with 1 level of perfect BAB progression (+1) and 9 levels of low BAB progression (+4), leaving him with only a BAB of +5.

I'm not sure of any official errata on the matter, but it makes a big difference in some cases. Ask your DM, or make sure your players know your ruling if you are the DM. Personally, I would stick with the latter "better progression" rule over the "better increase" rule, since it seems to be the intent of gestalt.

There is also the "Fractional Base Bonuses" house rule, presented on Unearthed Arcana p.73, which is designed to allow smooth leveling of gestalt multiclass characters without any of these exploitable loopholes.

Saves

Since you mentioned saves in your comment, I'll touch on it briefly:

Saves would indeed work the same was as BAB, but beware that they are even easier to inflate artificially if you use the first presented "better increase" rule. This is because at first level of every class with good saves, that save "increases" from +0 to +2. Therefore it's even easier to end up with ridiculously high saves through multiclassing gestalt, if you don't simple lump all progression levels together before calculating character stats.

As a side note, consider two classes which gain 1d6 sneak attack every other level. Staggering them as I suggest staggering BAB increases still cannot double your sneak attack damage, since the book explicitly states:

Class features that two classes share accrue at the rate of the faster class.

Duskblade 13/Crusader 1/Jade Phoenix Mage 6 seems to me to be much superior to Duskblade 20. You hit the really big duskblade class feature and retain your highest-level spells.

The issue is timing. The best levels to take Crusader are 5th, 9th, or 13th. Coincidentally, however, 5th, 9th, and 13th are also the levels where it is most painful to have a single level of crusader rather than being a single-classed duskblade, as those are the levels at which you get new levels of spells (and full-attack arcane channeling, in the case of 13th). You have two competing goals: get the next spell level (and full-attack arcane channeling) as soon as possible, while having maneuvers for as long as possible, since they are useful.

Therefore, I recommend taking the crusader level at 6th, 10th, or 14th; that is, I’d take the next spell level first, rather than the maneuvers. Here’s a breakdown of each option:

Duskblade 5/Crusader 1

You can take 2nd-level maneuvers with your first five maneuvers. This means, primarily, that you will have mountain hammer for the longest possible time, which is awesome. Other than that, though, you’ll probably still take a number of 1st-level maneuvers.

Duskblade 9/Crusader 1

You can take 3rd-level maneuvers and stances with your initial set; you can skip crusader’s strike for revitalizing strike, you can still take mountain hammer, and the amazing white raven tactics is available to you. Thicket of blades opens up as an interesting stance option; martial spirit is pretty small at these levels, and thicket of blades greatly improves your presence on the battlefield.

Duskblade 13/Crusader 1

Clearly if you haven’t taken crusader by this point, you should. Duskblade has little to offer, so you lose almost nothing at this point. The disadvantage of waiting this long is that you haven’t had maneuvers at all until this point. The advantage, on top of duskblade features ASAP, is divine surge, an excellent offensive maneuver.

The Awkward Bit

OK, so now that we’ve covered how to do it, there’s one other thing to consider: you cannot use arcane channeling and a martial strike at the same time. Arcane channeling requires a standard action attack or a full-attack, so the attack(s) you get from a strike don’t count. This makes strikes massively less useful to you than they would be to others. A boost or counter heavy selection of maneuvers solves this issue; you could do pretty well with defensive rebuke, shield block, and white raven tactics. But missing out on fantastic options like mountain hammer, tactical strike, the various healing strikes, and divine surge is a disappointment.

It also means you don’t have especially great mobility. You have invested 13 levels in duskblade to allow you to full-attack with spells, so you really do want to be making full-attacks. Crusader doesn’t help with that. Swordsage or warblade would help a little, since Tiger Claw is pretty good at that (sudden leap, arguably pouncing charge), but cleric for Travel Devotion or barbarian for Lion Spirit Totem (Pounce) might be better. Those options don’t lead into a great prestige class like jade phoenix mage, but they still might be better options.

The Other Thing

Finally, item-based healing is generally sufficient in 3.5. In-combat healing is rarely an optimal strategy, barring emergencies; the crusader is far better at this than other classes, but you shouldn’t usually need it. The aura you describe all-but-eliminates the worst emergencies, too. Wands and healing belts are quite sufficient for out-of-combat healing. As Jeor Mattan mentioned in comments, the real draw of actually having a cleric is the various options he or she would provide for removing status conditions, since items have HP-healing pretty much covered (and until you get heal, clerics’ HP-healing spells are pretty poor), and crusaders don’t cover that at all.

This is another situation where a level of cleric may be more useful to you: it would allow you to use wands of any cleric spells you like, with no Use Magic Device check. That will cover a lot your needs.