Two points, one overhead and the other open water! In open water my feeling as long as your breathing when you hit the surface you have enough air! Overheads a lot different! Requires planning and understanding of all the factors!
If a woman has a sixty and her buddy (male) goes in on 120 if stuff hits the fan with her no problem, but with him big problem because she has to have enough to get both out! She won't! In overhead you have to carry enough for your buddy otherwise go solo! If she has 120 and he has a problem then there would be plenty of air to get both out! A miss match in overhead does spell potential disaster! If you share usable are on the bottom I see no problem at all and if they end with 500psi its 500psi they didn't need!