2 boots, 1 ski

So it depends mostly on boot sole length and how much adjustability the binding has. Most bindings from the center of its adjustability range can be adjusted +/- 1 boot size. Warden & sth2’s have about 30mm total range. Marker griffons are about 20mm if I remember correctly. Pivots are 20mm officially but lots of people doubt that it is more than 16mm.

If you have 2 boots within the adjustable range of the binding you can have the binding mounted to work with both boots. The effective mounting point of the boots will be a little different but not a crazy difference. My old boots have a 326 bsl. My new boots are 318. I had a pair of skis mounted while making the switch and intentionally had them mounted for my old boots to be on the recommended line and the new boots would be about 1/2 cm in front of the line. I did this knowing that if I hated the new boots and went back to the old boots I would not want them to be mounted behind the line.

As long as the binding and boot are compatible and adjusted correctly, you can change your boot/ski every day if you'd wish to do so