Hi!
If you can get a good amount for it, I suggest you to sell the 970 and buy a new PSU or get a better cooling for the Titan X, like the Corsair HG10 N980 (that will be released in August), plus an AIO like Corsair H110iGT (around 200$ in total)... or the EK Titan X if you already have water cooling on your pc.
Nobody knows for sure if you will be able to play games with both cards... or if the developers must implement that posibility on each game. So, if you keep your 970, there's a risk of loosing money because it looses value over time.
On the other hand, by selling it you could get a better PSU and what I said before... that would suppose a safer destination for your money.
And finally, DirectX 12 will improve everything of your rig and the Titan X will be even better than it is, so you won't probably need anything else to play at good resolutions and you would be able to overclock it by purchasing what I said.
Good luck with your decision.