Yes, two 980 cards will definitely outperform one 980Ti or TitanX card. Basically $1500 vs $1000.
Two 980Ti cards are roughly equivalent to three 980 cards. You could always buy one 980Ti (and a beefy PSU) now and add another later.
I run two high-end GTX980 cards on a single 2560x1440 ROG Swift. It takes a *lot* of load at absolutely extreme/max graphic settings to have my fps dip below 60fps for even a few moments running any game or bench. Much overkill for single-monitor gaming, I think.
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