Achievement Stacking Question

Essentially I already started on the X/S version before I started on the One version and then when I realized about the stack I started on the One version and am nearing completion.

Will I still get the stack if I upload my One save to the X/S version although I have save data on the X/S version?

I'm not 100% sure on this, but I would think that you will still get the achievements from the stack. For single player, you could wipe your save on the X/S version before transferring your last-gen save just to be safe, but I'm guessing that wouldn't even be necessary.

For multiplayer it would probably allow you to download your Xbox One character in addition to whatever one you already have on the X/S version, though I believe you can only have two characters max so you may have to overwrite one if you already have two on the Series X/S.

FYI you could also stack the Japanese Xbox One version if you want while it's on Game Pass; most of the MP is pretty quick after doing it in the regular version; both Xbox One versions share multiplayer progress but requires you to redo some stuff, but it's not too time consuming. Though you would have to do single player again.