Set Forced Rendering Frames to 2, that forces it to use sli and see if you get the same numbers. If you don't and it works, then the driver you have is not detecting FFXIV as a SLI game. In which case you should Get GForce Experience or w/e it's called and check if it has any new game profiles. Also make sure you are using the latest stable driver.
Also, I could be wrong here, but I'm pretty sure SLI doesn't kick in for heavy use until it's needed. It's quite possible your 1 770 can handle the load and isn't giving the 2nd card much work to do. With a 770 gtx your bottleneck will be your CPU. If your CPU can't work fast enough to load the card you will lag, but it's not your GPU causing it.
Think of it like 1 boss and 10 workers. Let's say there is enough work to do for 9 workers, but your boss is only capable of coordinating it for 5 people. In which case you see the obvious, there is work to be done not getting done. So you will think your workers are not good enough and you need more. But in reality you need another manager or boss capable of allocating all of the work to the workers efficiently, or to replace the boss outright with a better one.
If your CPU is maxing out on all of it's cores (the boss), it's not capable of loading the GPU (the worker(s)) fast enough, so they are under load, you see some lag here and there, but you don't need more GPU's you need a better CPU. You need both, just upgrading your GPU isn't going to work forever.
For an MMO the best upgrades you can buy is
-a Blazing Fast CPU (higher the clock speed the better) e.g. I5-2500k overclocked to 4.8 ghz (you can get these under $300)
-an efficient powerful GPU (good workers) like your 770.
-A really good nic. The nic is what drives all the triggers that run the game, Killer Nics are decent but overkill. Some onboard nics like some realteks are complete trash. Onboard intel nics are pretty good. Ideally you want your nic to do as much of it's own processing as possible to prevent the CPU from picking up all of the load. Cheap nics make the CPU do everything, good nics do a lot of the processing on the Nics CPU. And absolutely do not run WIFI, you are gimping yourself if you do, Unless your on a laptop hanging out in your hammock in the backyard, use a nic and an ethernet cable.