You'll be fine. The entire framebuffer -- including your desktop -- is duplicated in the local memory of both cards, so both cards will carry the (very, very minimal) burden of showing your desktop.
Most MMOs these days -- and even some other non-MMO games -- support "Borderless Window" or "Fullscreen Windowed" or "Fullscreen Maximized" mode. I STRONGLY advise you to use this mode, although it may or may not play nice with SLI depending.
Bleh, this is why I don't use SLI anymore. lol. (⊙﹏⊙✿)
i5-3570K @ 4.4 (NH-C14), 4x8GB DDR3-1866, GA-Z68MA-D3H-B2, ASUS GTXTITAN-6GD5, 128GB Vertex 4 / 2x60GB Vertex Plus R2 / 2x2TB Barracuda 7200.14 RAID0 / ANS-9010 (4x4GB), SST-DA1000 (PSU), 2x VS229H-P, 1x VG248QE, 1x MIMO 720F, Corsair Vengeance K90+M95