You mean, a helmet and chest from the same set?
In that case, you could always do like this:
Create two form lists for each set: one helm, one chest.
If there is, for instance, 3 armour sets, create 3 functions in your Actor Effect form(or whatever you're using) which all enable waterbreathing. Then create two conditions for each function, with and AND function for the two forms(helm, chest).
http://i485.photobucket.com/albums/rr219/Naima_den/example.png
(Woops. Replace 'IsInList' with 'GetEquipped'.)
This is what I mean. Each Waterbreathing function has two conditions for each armour set.
Hope this could help.