You are going to need a 360 rad to cool that...most 360's will handle around 500-575 watts with 1500-1800 rpm fans. Even an RX240 is only going to get you in the 375-400 watt range with 1800+ rpm fans (360 rad handles 575-600 watts average, x 66% = 375-396 watts; 240 rad is 66% (2/3) size of the 360).
Either way, looks like a single 360 or a 240 and 120...you can keep this as a single loop, no issues.
When it comes to water cooling and especialy custom built water cooling you can pretty much design your own system to whatever you want. The main question is space and where do you want to put the radiators or rather where can you put them. If you want to put everything inside the case then you are going to be tight on space , if you want to put some of it on the outside then you open up more space to use. Then you come to using the desk as well as the case to put things and it helps if you have a nice big desk.
Now in your setup you want to cool two video cards and the cpu and you should be able to do that with a 360 radiator , now if you are going to overclock any or all of those components then you will have to increase the radiator to a 480. If you then decide to do two loops then you are talking about two radiators , two resivoirs , two pumps and more tubing.
Also if tou add any other components like the ram or the motherboard chipset then you will be doing two loops.
You don't have to do 2 loops, regardless of the loop size. You could simply run 2 pumps if you are concerned about flow. Otherwise, having dual loops is a waste of resources- even if you ran a hybrid dual, you can still use the same res for both loops and would perform better than separate dual loops.