When you multi-class do you normally use one character sheet or do you start a whole new one?
For any D&D 3 descended system, multi-classing is built in as an idea, so character sheets are usually designed to be one unified document. Starting a new character sheet in that would be weird, I'd immediately be suspicious of a player doing this.
I guess in AD&D, original Palladium (that still allowed that) and some old school revival systems it might be a bit different; but I don't think I've put any significant play time into a game that allowed multi-classing but didn't orient character design around it appropriately.
I'd say use one character sheet. Most systems that have multi-classing account for it by design I find. Though if it doesn't fit on one sheet, I'd suggest going online and looking for some of the fan-made sheets that /usually/ have a lot more room and ease of use to them than the ones the game itself provides.