Thanos can't win.
1. They're so fast that he would never see them.
2. Thanos most impressive attribute is his durability, which to some extend is based on his force fields and his battle armor, which start to crumble under too much damage and Flash and Zoom have the potential damage output to cause that damage in a short period of time.
While I don't see one of Zoom's "pre DCNU Superman level punches" being as strong as a Mjolnir strike by an empowered Thor, I can see hundreds and thousands of these punches in a second having an impact on Thanos.
Now, don't get me wrong... I'm not saying that Thanos isn't powerful enough to take few of Flashs / Zooms infinite mass punches... They're pretty damn overrated, but this is now a combination out of Thanos not being able to make a move against characters that fast and the ridiculous amount of PIS that is the Speed Force and the Negative Speed Force that empowers speedsters and protects them from everything.
The fight would be over in the first attofemtopiconanololotrollolo-second without Thanos ever getting a chance to do anything at all.