I actually have a PR to set up mapping between player controllers and devices (so you can control more player controllers with one keyboard, control player 1 with gamepad 2 and player 2 with keyboard, or whichever combination you like): https://github.com/EpicGames/UnrealEngine/pull/5209
If you have a source build project, feel free to try it out and give feedback if you find anything to be broken (or if it works for you)!