How do you target the target of your current target (assist target) with a controller? The default keyboard/mouse key is T.
As far as I know, there currently isn't a button mapping to assist-target directly via controller.
However, the same can be accomplished via a macro, which can then be assigned to your cross hotbar:
/as) will assist your current target.
/assist "Player Name"(or
/as "Player Name") will assist a player by name.
/as <2>) will assist the party member in party slot 2. (Useful if you tend to re-order your party list to put, say, the main tank in a specific slot.) You can change the number to any other slot, as well.