How soon can I upgrade a cage?

[Elleonor]

Hello everyone,
I got a pair of baby male winter whites about a month ago (so they are 8 weeks old now). I originally planned on only getting one, so the bin cage I made was supposed to be enough for at least a few months. It is a 30 gallon bin. Attached a pic of the original setup.

The plan was to wait while the hamster is fully grown to upgrade him to an IKEA Detolf. However, now that there are two of them, just putting in another wheel limits the space immensely. I get the feeling they would really like more.

How soon do you think I can upgrade their cage? I don't want to cause any stress to them by switching homes too often.


Additional info:
Both cuties became fully tame within a week and they are very friendly with each other. The most hostility I see from them now is shoving to get to treats, water or my hand. They sleep together and play when free roaming around the room.
One of them had a hard time at the beginning, though, and didn't come out of the tunnel for two days. It also took time for him to adjust to free roaming.