I think this is probably not right. If you always have exactly one bottle open, the magnum will always have more total surface area. If you drink less than 750

I think this is probably not right. If you always have exactly one bottle open, the magnum will always have more total surface area. If you drink less than 750 ml from both, yes the ratio of area/volume will be smaller, but conversely it also means you are tainting more wine. And if you wanted that effect, you could always open the second bottle the next day and mix the two regular sized bottles.