This looks like the perfect device for this activity:
It's an aluminum funnel that uses a clear tube to connect to an aluminum tunnel butt plug. I don't know what the tube is made of. I am not affiliated with the site at the above link. Use the product above at your own risk.
Anyway I'm sure one partner could put a funnel on one end of an enema hose and put the nozzle at the other end of the hose in the receiving partner's ass. Just a speculation, however, so if you experiment with this then do so at your own risk.