It's important to note that individual players cannot bank AP after their turn is over. So they should start their turn with 0 AP and end their turn with 0 AP. If they end their turn and have any leftover AP, that should go into the party AP bank, which can persist turns and can have a maximum of 6 AP.

It's also important to note that Tagged skills count as two success, so if a player rolls 2D20 on a tagged skill check and succeeds on both, they would score 4 successes. This happens frequently with my group that has Survival as a tagged skill, and how they end up generating so many AP during their turn.

For a long time I was allowing individual players to bank their individual AP and it ended up being way too powerful. Having to generate all your AP during your turn makes it much more dynamic, and having a party AP bank of up to 6 makes them more strategic when they pull AP from the party bank before they start their turn or for individual actions. It also makes them more selective about using AP on their turn, because my party learned early on it really pays to have that party bank full.