Of course that could be an issue, I can speak only from my personal experience. I have belt tensioned exactly according the manual, so its not overtensioned and no slippage there at all. I think that if you have new belt with correct V and with good surface (not a cracked, hard and polished one) it can works even with single pulley. You would be surprised how much power can one belt transfer.
But Im not questioning that double pulleys are more proper and nicer
