Is this all going into a Corsair 540? If so, just fill it up with a 240mm top and 360mm in the front. That should be fine for a CPU + 2 GPU loop.
Our usual rule of thumb is for every 120mm is 100w. So you're looking at a 250w GPU with 200w of heat surface. It can get the job done regardless, it all comes down to how efficient you want your loop to perform.
the general rule of thumb is 120 worth of rad for each 100 watts of tdp. As the titan xp is a 250w tdp card, 2 of them would have a tdp of 500watts so you would want 5x120 worth of rad to cool it. You could probably get away with 4x120 worth or rad but i would not recommend trying to run 2 of them on a single 240 rad.