It looks like THIS/THAT can either be grouped using the pre-set pairs, or you can split them and assign different behaviors from the various Selector modes. So, based on my reading of the docs, you should be able to accomplish what you laid out in your post. Looks like, once you’re in the Selector, you press and hold an initial mode which is assigned to THIS, after which you assign a different mode to THAT. I think 
EDIT: See @voidstar’s clarification below!