I read something about this a few days ago. According to reports, there are plans to put the tag titles on Mysterio & Cara as part of a build up to their WM match. If that is the plan, then I'd rather see them have a fairly long run with the titles, such as maybe winning them at the next ppv, rather than have a lame duck run in which they just have the titles briefly and drop them.
As far as a heel turn for one of them goes, I have to admit that I'm a little intrigued. Mysterio has been a pretty beloved babyface for a long time in WWE and his merchendise sells well, especially among younger fans, so I can understand reluctance to turn him. At the same time, I find myself curious as to whether or not being a heel could inject some real personality into him. Mysterio is pretty bland in terms of personality so maybe a heel turn could be a pleasant surprise. It's not something I'd bet on, but I think if Mark Henry can become as interesting as he became, then I certainly think there's a chance for Mysterio to be an interesting heel.
What I could see happening revolves around something simple, like maybe one or both of them becoming somewhat envious of each other to the point where it gets a little competetive. They try to show up the other during their matches. After all, Mysterio has been the only luchadore in WWE for a very very long time and maybe he could feel "threatened" by Sin Cara. Ultimately, the suspicion leads to resentment that could lead to a double cross that results in Cara losing the titles to opponents down the line.
If Sin Cara has managed to get a grasp on English, then I could see a heel turn for him. The WWE could go with the angle of him being the young, up & coming guy that feels the aging Mysterio is holding him back, that he's not quite what he once was and that it's time to step aside. Sin Cara is much blander than Mysterio in terms of personality and he's just not connecting with audiences like Mysterio has. With the way things currently stand with Sin Cara, with him being the faceless & silent masked man from Mexico, he can only go so far. If he's not ultimately able to connect with fans as a character, then he's not going to ultimately be able to take Mysterio's place no matter how much the WWE might want it.
Keeping the match as a competetive, face vs. face contest could work as well. I don't look for this match to garner a ton of attention in terms of build or hype. Reportedly, the WWE is talking about Triple H vs. Lesnar II happening at WM or Taker vs. Lesnar. It's expected to also be Cena vs. Rock II at WM, but there's also a possibility of putting them in seperate matches. That all ultimately depends on whether The Rock will be free to wrestle. At any event, Mysterio vs. Cara won't be a major money match for the show, so the WWE could get away with keeping both as a face without the worry of fans not being interested because keeping them both face would probably mean a lack of character development for one or both.