We do this a couple of times a year from the Phoenix area, where sub freezing weather is unusual, to Flagstaff, where 0 to teens are pretty common. Not many in our troop have bags good to 0,(typically ~ 40F) so we tend to use a double bag approach. Very rough rule of thumb: subtract the temp. rating of the warmer rated bag from 60, then subtract that difference from the lower rated bag. For your bags, that would be (60-40) = 20, subtract the 20 from your 15, and you should be good at 0. If you sleep cold, add 5-10 degrees, and you should be good at that temp. Your pullovers and vest would be insurance / daytime warmth.
Hope that helps,