I’m currently traveling internationally with an iPhone 12 and I can confirm the single eSIM + single physical SIM limitation. Although, in my case, I'm using a physical international SIM and a US eSIM.

I would love to turn off my US eSIM when not in use (I think it uses more power connected to two cellular networks) but that would require unenrolling my US iMessage number and I can’t do that. Definitely the most annoying part of the whole thing.

I considered using a spare iPhone to host a physical SIM with my US number because that would allow the number to stay bonded with my Apple ID and potentially forward SMS over iCloud, but I decided not to because in my experience the SMS part is too flaky to be relied on.