i would say no in a 20 unless you can rehouse whichever female he doesn't want.
When i had rams some years back i was shocked by how long it took them to pair up, like 8mnths or something. And it was only after i moved them to a different tank that they decided to get jiggy.
You could try removing one or other of the pair you currently have and reintroduce them, after a week or two without seeing each other they will be strangers.
Hope i was of some assistance.
