We have that problem with Sally and Spud. It has taken me a year to teach them that they need to take turns, and they still don't have it down well. I'm teaching #1 to sit/stay while I throw for #2. Then switch them out.
I work with them for a little while like this. Then I release them and let them retrieve for fun, which means the whole "one take it away from the other" scene. So, my solution....get two balls. Throw one in one direction....dog #1 goes. Immediately throw the other in the other direction...dog #2 goes. So both are retrieving at the same time.
Tal and Barney go after the ball at the same time. Most times Tal gets it, but sometimes Barney does and then its on! lol. When Tal gets tired, he just let's Barney go after it, bring it badk and then takes it away, brings it to me and here we go again!