Yes, but can one make more than one unbreakable vow, assuming they're not mutually contradictory?
I would think that if Voldemort had any doubts about Snape's loyalty, ol' Severus would have been toast. The Dark Lord doesn't seem to be much of a chess player compared to Dumbledore - much more the blunt object. It seems, based on Bellatrix's grilling of Snape at the beginnning of HBP, that Snape and Dumbledore anticipated Voldemort's questions and reactions.
I'm sure one can, but there's something inherrently dirty about the unbreakable vow and I don't see Dumbledore as doing that. Since Dumbledore actually believes in loyalty and love and the basic goodness of most people I think he'd rely more on explaining his view of the situation to Snape and his goals in it and letting Snape do the right thing.
The reason I think Voldie wouldn't just kill Snape outright is that Snape is in a useful position, he's just too useful a tool for Voldie in Dumbledore's inner circle. Voldie doesn't really trust anybody anyway, I just think he trusted Snape less. Voldie isn't a chess player like Dumbledore but he thinks he is, he thinks he's smarter than everybody, thus he would think that Dumbledore is too dumb to detect if Snape is really loyal to Voldie and would think that if it turns out Snape is really loyal to Dumbledore he could handle it.