I don't think the attempt on Michael was ordered after the meeting of the Dons was set up. I think the hits on Michael and Sonny occurred just a matter of days apart. Sonny was killed, Tom set up the meeting and likely got word to Sicily that Sonny had been killed. When we see Tommasino give Michael the news it could have very well been the day after the toll booth shooting. The car bomb could have been the next day. If that timeline is correct, both hits were in motion BEFORE the meeting. Barzini had to take out both sons because they both would likely have come for revenge. He wasn't worried about Fredo for obvious reasons, but Sonny and/or Michael would have sought vengeance for the attack on their father.