The change of quickslots is also available duing combat. But you manually need to press the "use" keybinding!
So you could use e.g. quickslot helper addons like FCOStarveStop (auto switches to a chosen quickslot during combat/out of combat, depending on dungeon/AVA, missing food buff or if a potion runs out [as a reminder]), Static's Quickslot Profiles and Greymind Quickslotbar (the last 2 provide keybindings to manually select a quickslot, e.g. use F1 to F8 for the quickslots.
F1 is your potion 1, F2 the other.
FCOStarveStop reminds you as the potion is usable, and you'd only have to keep track/remember yourself if last potion used was the F1 or the F2 potion.
Press the other key then (if last was F2, perss F1), and after that the use keybind. Next time, press F2, after that F1 and so on.
Also helps to quickly switch to e.g. health potions at e.g. quickslot 8 and so on.
Before you ask: I won't add anymore logic into FCOStarveStop
So sorry, this would be all that could be done for the moment.