I believe in certain dx 12 games it would, other then that i don't think it'd work period. Also from personal experience most games don't support sli setup's so go with the best single card you can afford.
Regarding the sli i believe so, it would be a shared pool of memory, actually no. Now that i think of it those are two completely different cards. The micro arch for the 980 ti is gm200 and i believe 980's are only gm100. So honestly i don't even know if they'd work at all to begin with within that niche market of games. If it did again shared pool of memory so 4gb of gddr5 ram, it would have to match the same speeds as well on top of that.