If Cain goes for the double leg takedown, I think it would be possible that he takes JDS down. Both Cain and JDS fight in the orthodox stance, where there is a closed guard and their hips are much closer, making it easier for Cain to go for a double leg. JDS actually got taken down by Gonzaga who utilised a double leg takedown, though Gonzaga wasn't able to keep JDS on his back. Mir went for a double leg, but switched to a single leg and didn't manage to score the takedown. However, Mir's wrestling is mediocre and Mir fights in the southpaw stance, which leads to an open guard and forced Mir to cover a longer distance to reach for JDS legs. JDS eventually escaped from Mir's single leg. Cain, who has superior wrestling, might be able to perform the double leg takedown better.
With all that said, JDS is fast and might be able to back off before Cain reaches the rear leg, presenting Cain with only the lead leg to hold on to. I'm unsure of how good Cain's double leg is though. He went for single leg takedowns against Rothwell, Lesnar and JDS in the 1st fight. He got a few double leg takedowns on Rothwell in the second half of the fight, but Rothwell has way inferior TDD defence as compared to JDS.
I don't think a single leg takedown would be enough to get JDS on his back. Single leg takedowns are much easier to fend off and JDS has great balance. Would have been great to see JDS vs Lesnar. Both fight in the orthodox stance and Lesnar has a strong double leg takedown. Might have been able to see more of JDS on his back.
Anderson Silva ♔
Aldo - Dos Anjos - Mayday - Weidman - Machida - Jones