There are several reasons why you might want to cast True Strike. The first is that depending on your class, you might be able to get a bonus action attack after casting it, giving you one attack with advantage and one without, a definite improvement on two attacks without.
The second is that you might use it to overcome disadvantage:
- When you need a die result of 9 or higher to hit, a single normal attack becomes more likely to hit than 2 attacks with disadvantage, so it's worth the trade-off of using True Strike.
- When you need 8 or below, though, two attacks with disadvantage are more likely to hit, so just attack away without True Strike.
The third reason is that you might want advantage for something. For example, a Rogue might use True Strike to grant advantage to allow use of Sneak Attack.
Another reason is that you might know in advance that there was going to be combat, and cast True Strike for advantage on the first round. This would probably only happen when ambushing, but is still worth mentioning.
The final reason is that you might have an attack that you particularly wanted to hit. A Wizard using Plane Shift to send an enemy to the Abyss, for example, would want to be certain that the melee spell attack required would hit, otherwise they've wasted a high-level spell slot for nothing. Casting True Strike first would make it considerably less likely to miss.
The way I've interpreted this is that the spell itself, the casting of it, takes the duration of one round, your current action. The effect itself does not take place until your next turn as stated in the spell text proper. Once your next turn begins the spell is active, so to speak, and for the duration of that round you gain the advantage listed.
Furthermore if concentration is lost between casting and actual use, you'd resolve it as you would normally for checking on a break in concentration.
TRUE STRIKE
Divination cantrip
Casting Time: 1 action
Range: 30 feet
Components: S
Duration: Concentration, up to 1 round
You extend your hand and point a finger at a target in
range. Your magic grants you a brief insight into the
target's defenses. On your next turn, you gain advantage
on your first attack roll against the target, provided that
this spell hasn't ended.
So cast spell, wait for next turn, spell effect is active for that turn, profit.
Furthermore I'd treat an attack on that turn that a PC might be able to make as a bonus action as the "next turn" attack with advantage, aka the first attack roll against the target mentioned in the spell proper. But the text is pretty clear that it starts working on your next turn, so that would likely be up to the DM.
Best Answer
You add it to both
Perfect Strike allows you to roll "your attack roll", the thing True Strike modifies, twice; It does not cause you to make two separate attack rolls. Since it is a single roll rolled twice you get the +20 to both rolls. In the odd event you roll a critical threat with one of the rolls you still add the +20 to the other roll that is now your critical confirmation roll because it starts off as 'your attack roll' then gets used for something else.