That is usually rare case, because in most games even if there multiple heroines each one have a route dedicated to her. If talk about swapping, then this is basically any NTR with swap. xD
If you want a harem, there is NTRed-harem in some RPG-NTRs for example
http://www.dlsite.com/maniax/work/=/product_id/RJ176529.html
and
http://www.dlsite.com/maniax/work/=/product_id/RJ211965.html
Also don't forget Makenshi Leane, I think it can be perfect for you!
BTW authors of first one have new game released, I NEED to find and play it. ConfusionWorld from them was one of the best NTR-RPG for me, because of their sandbox like gameplay and partial voice acting.