I'd think it would be a cool idea. The only problem i'd see is Rey playing 2nd fiddle to CM Punk. As we know (with the Ziggler incident last year) Rey does see himself in that 2nd tier in WWE, which is probally where Punk is too.
I'd be happy with the Idea of after Mania of Punk teaming with another bigger star (could be Rey) and becoming the Tag Champions (Say Morrison & RTruth win) so he could be on both brands coz i'm loving his preacher type gimmick and i think he's a good champion.
I can't see it happening but i'd love to see Punk on both brands for a bit