You want the 2 slot bridge part number ending in 21, since it's SLI with one slot spacing. If you use the 3 slot bridge, you'll have to either put the second card in slot 2 with no spacing or all the way down to the fourth red slot with double slot spacing, and either one of those means you'll be running on the PLX bridge chip, which IMHO has something worse than the latency it adds...if you actually use the PLX bridge, it heats up a lot, can get over 60C on my system, and that's a case with excellent cooling and stock Nvidia Titan-style blower fans. Gets worse if you're using ACX coolers, since there's even more heat dump inside the case.
TL;DR: Use the black slot and learn to live with the skinnier pro bridge (or order two!), or use any combo of red slots, add heat, drop performance, but get a better looking bridge.