I think it is just going to take some more time. Hopefully they will come to be the best of friends and soon. That would be amazing. It will just take time, patience, persistence and consistency play time with both but also especially with one who may seem a little more shy. It builds their confidence because to them play is hunting.That's exactly what I'm imagining! Something heavy enough to just prevent them from moving it should suffice. Pip is more of a freeze in place or flee kinda guy, he's never showed interested in wanting to fight Arwen, he just does more of the I-don't-like-this-get-away-from-me hisses or otherwise he'll flee. So thankfully I'm not too worried about them fighting necessarily, I'm more worried that Arwen will unintentionally bully/terrorize him just by not knowing how to take a hiss for an answer lol.
I believe things are going to work out great.
You will have to let us know how the room divider works out. I'm imagining it to work well.