Congrats on the new speakers, Roidefromage!
The RCA splitter should work fine. I run multiple subs and that's what I use. As for placement, well, that's up to you. Try a few different combinations and run some frequency sweeps with an SPL meter to find out which works best.
For me, I like the subs spread out around the room for HT. I have one in the front right corner, one along the left wall, and the third right next to my seat. For music I preferred the subs up front next to the speakers.
Have fun and experiment.
Don't worry about speaker burn-in. If anything changes it will be your ears and brain adjusting to the new sound.
