First Cooperation?

I really need to have the video camera nearby. So, one of the funniest things to watch with twins is the battle over a single pacifier. If they are sitting near each other, one will pull it out of the other's mouth with a "pop" and put it in her mouth. Then the other will reach out, grab it and and pull it back out with a "pop" and put it back into her mouth. This continues back and forth, and usually (and surprisingly) without either one getting too upset. Well this morning we had another pacifier battle, but this time after a while, Zoe took it out of her own mouth and reached out and gave it back to Zadie. It seemed as she thought this was more of a game rather than a posession battle. Is this the first step in cooperative play?