not sure but I have heard the number 300 thrown around as the total, not sure if it would divide the weight equally. I'm thinking maybe the bibs would float 2/3 and jacket 1/3 just by the amount of material that is in each.
That said I would rather have my upper body floating than just the bottom half, if you go unconsicous at least your head would stay above water. With just the bottom floating they may find your dry boots floating high and dry and your head underwater, which would not be good, LOL
J-