To help your dog stay on doubles set it up so there is a barrier between the throws like a corner of a house, fenced yard etc so that when he turns to see the next throw he does not see the first one down.

Have your dog securely tied to something behind you with just enough slack so you can walk him forward so he cannot see that first throw, have a quick release snap. You do the 1st throw with a bumper to your right. As soon as he settles even a millisecond have the other thrower yell and throw the BIRD and release him immediately to retrieve this bird. When he returns take the bird and line him up for the second retrieve and send him for it, use a white bumper and walk toward it if he has to be reminded of it. You are teaching him that he will get a second bird only when he settles down so it is his choice, the quicker he settles down the quicker he can get a bird. The second one thrown is more rewarding than the first which is only a bumper. Do not try to correct him or say anything other than "mark, and stay", that's it, let him figure it out.