The game reads conditions, left to right, and goes with the first one which is true or doable.
In the case of: /cast [help, target=targettarget] Barkskin .... this is only true if the targettarget is the casting player, because barkskin is self cast only.

If you want Heal on another target or barkskin on yourself, go with something like:

/assist [target=party1]
/cast [help] Heal Spell; [help, target=targettarget] Heal Spell; [target=player] Barkskin.

Clicking this and your toon assists the party leader (or the current focus (ie /assist focus) if that is how you determine assists). If the target is friendly, it casts the heal. If the target is not friendly, it moves on to the second target sequence, which is the heal spell on the target of the target. This will almost always return as true, because your party leader will be targeting a hostile who in turn targets someone hostile to them but friendly to you. If both of these conditions are false (ie, assist's target is hostile and that creature's target is also hostile to you), it defaults to you casting barkskin on yourself.

To have a spell cast Barkskin on yourself but another spell on someone else, you need a logic arguement where the (targeting for the) spell to be cast on someone else will not be true when you are the target, but will be true for someone else. I'm not sure how this would work, but you'd need to use logic to that effect.