Yes, this is super inconvenient, but any (!) part of idle can only be played from one instance. Even when you use a Multi-Level power, playing those two idle levels on two devices will (probably) cause issues.
The problem here is less that the game is snapshotted but more - so is my understanding - that the second instance isn't syncing back its state before the first instance ends the battle. So in theory, if you started the battle on device A, then on device B continued your level, then before the battle ends forced a sync on device B and preferably closed the idle instance on device B, then on device A all the actions that you took on device B should be present. This is by no means a recommendation!!!!! I can't guarantee you that this would work and I will not test this due to the risk to brick my idle progress to some extend.
Also, I don't know, whether the battle outcome has to be synced similarly, therefore, it is absolutely possible that this strategy results in a corrupted state itself.
The gist of this all:
Edited 7/10/2021 00:50:31
- The game should more directly inform the players that this is strictly a single-instance mode. The only real information about this is the forum - that you will most likely only consult if you already encountered the problem - or the wiki - that frankly, most of the players won't even know about.
- Don't stress your luck and always play from one device at most at any given point in time.
- Before you switch devices, I always force sync. When I do so, I never lose progress. When I wasn't that strict about how to switch devices I once lost a hospital upgrade I think. Nothing detrimental, but still unpleasant.
- And finally, let's hope that it is possible to, at some point, play idle from multiple devices in parallel. But I guess that while idle is in beta there are far more important things to do for Fizzer.