After a bit of tinkering I managed to get SLI working using Nvidia Inspector. This solution is not perfect as some surfaces flicker when effects quality is set to high or turbo in game. Every other setting can be maxed out without issue. I have only tested this with dual GTX 970s in SLI so I have no idea how this will fair with different hardware configurations. I now get a solid 60FPS in 4k versus 30ish before.
For those that are familiar with Nvidia Inspector all you need to do is set the DX11 SLI bit to 0x080222F5 for Tower-Win64-Shipping.exe.
Otherwise here is a short guide:
Download Nvidia Inspector Here: http://www.guru3d.com/files-details/nvidia-inspector-download.html
After running it click the wrench icon to bring up profile settings:
Make the changes below:
SLI bit - 0x080222F5
After launching the game ensure that effects quality is set to medium or lower. There is some minor flickering with the high setting but its still playable in my opinion.
As you can see both of my graphics cards are being utilized now:
This SLI profile is designed for Batman: Arkham Orgins (Another Unreal Engine Game). SLI profiles need to be tailored to the specific graphical features a game contains. There is no one size fits all solution for Unreal Engine 4 games, but luckily Arkham Origins is close enough to Tower Unite technically for this to work. This may change as Tower progresses however.