This is extremely engine specific and not really a general statement that you can make, I don't think. Its quite easy(engine wise) to pack 2 256x512s into the space of 1 512x512, its not like you're using 357x932 instead of a 512 or something weird like that. But again this is highly dependent on how your engine loads…
To do objects like this I start out with a box, unwrap it in two strips. Then apply turbosmooth (max modifier), then sphereify to fully round it out, this turns it into a quad sphere, I suggest making a primitive shape of this because you might end up using it a lot. Because you unwrapped it as a box you now get the nice…