Play with different locations and listen. Plugging them into a long power extension cable facilitates the experimentation. Best location is what sounds best to you. 

You might want to look for some guides to setting up multiple subwoofers (not just Sonos) in room, it can be challenging to get even sound with no hot or dead spots.

That may save you trying unworkable positions but as said above trust your ears.

Might consider both under a couch, laying flat, an option many subs can't offer.

Also for do this faster you can use a technique (I don't remember how it calls) but consists in put the Sub in your listening position and you going with your head at the same height of the Sub going side to side searching the best position and once you have a position of one of the Subs do the same but now with one already placed well and another equal to the first and going searching, but do this with a music, film or something you know well the sound, and after run the Trueplay :)