Ah I mean more of having Immunity to only Fire and Immunity to only Electricity with Supernatural Toughness (catch: water); that can all be researched. So it would sort of look like this:
Supernatural Toughness -4
Catch: Water (+3)
Physical Immunity (Fire) -8
Catch (Only Fire) +5
Physical Immunity (Electricity) -8
Catch (Only Electricity) +5
Though perhaps I’m wording this wrong, I essentially am asking if having immunity to Only Two things (in this case fire and electricity) possible.
You're doing that wrong. The actual structure would look like this:
Supernatural Toughness -4
Catch: water (+3)
Physical Immunity -8
Stacked Catch: anything that's not fire or electricity +N
Now, the only question is, what's the value of N? Specifically, does it get the +2 for only protecting against fire and electricity, or does it lose that since that's two things rather than one? Were I GMing for you, I'd probably rule that fire & electricity is a sufficiently limited category of stuff for that catch to still be worth +5, but other GMs could easily argue that it's worth +4 or +3 instead (+2 for easy to get stuff that's neither fire nor electricity, +1 for researcheable, and maybe +1 for only protecting from a limited category of stuff).