You have received some great training tips.
I would like to add one.
When you first start out place a pile or 2 or 3 of feces from each dog in their potty area. They will smell that there scent is already there & will see that it got moved which should make them think more so that that is the proper area to eliminate themselves in. This should encourage them to go at first.

I use "go potty" for my potty phrase.
It is especially helpful too when traveling as you teach your dogs to go on command. And it comes in handy if for some reason you ever need to get a fecal or urine sample for your vet.