This is how we ran it in an OD&D campaign I played in last year. Each adventure I had to choose which class my xp would go toward.I find the OD&D elven switching rules hard to parse. Switching from one to another on a day seems to make sense on its own, choose to be a fighter on one day and be a fighter and not a magic-user that day, or choose to be a magic-user that day and not a fighter, but then that seems contradicted by the next line of gaining both benefits including specifically wearing armor and casting spells which seems to be at the same time. The best I can come up with is that you can do both classes' stuff at any time but only gain experience and levels in one class on any one adventuring day. Would HD stack? Would they start out single class, day two switch and gain a new HD and class features for being 1/1?
I could use all of my abilities but not cast spells while wearing nonmagical armor. I rolled hit dice separately each time I gained a level, but only changed my HP if the new total was higher. I made it to 3rd/3rd, IIRC, before the DM had to put the game on hiatus, and hadn't found magic armor yet. Usually on a given adventure where I has chosen M-U I would enter the dungeon unarmored but with a hireling carrying my plate in a pack, and once my spells were cast we'd hole up in a room and I'd spend a turn donning my armor.