Has anyone used a local data SIM with the eSIM simultaneously?

That is. you have eSIM for your domestic network, then you travel overseas -- which should be more likely in the near future -- and you buy a local prepaid SIM for mobile data.

My understanding is that iOS allows dual SIM mode so you would be able to receive calls on either number, the eSIM or the nanoSIM.

But you'd only have data access from one of them at a time.

Is the switching automatic or will you have to explicitly toggle the cellular data?

OK, currently have a T-Mobile eSIM on my 12 Pro Max, iOS 14.5.1. Under Settings->Cellular, I see a master Cellular Data toggle and then T-Mobile with the "Turn on This Line" toggle. Then in the next section, I see Add Cellular Plan, which when tapped offers to scan a QR code. Isn't that only for adding eSIM and I already have an eSIM provisioned.

When I tap Enter Details Manually (in Settings -> Cellular -> Add Cellular Plan -> Enter Details Manually, it shows Enter Activation Code with fields for SM-DP+ Address, Activation Code, Confirmation Code (Optional, but required if provided by your carrier).

Presumably if I insert a physical nano SIM, it would get added to the section below the T-Mobile SIM.

But how would I make sure it's using the mobile data from the nano SIM while still being able to roam for voice and SMS with the T-mobile eSIM?

Because obviously would prefer to use the T-Mobile number setup for FAceTime, iMessage, WhatsApp, Viber, etc.

OK this page covers some of the UI for managing dual SIMs:

https://support.apple.com/en-us/HT209044

Still would like to know about adding a physical SIM and how that would appear. It shows some screens which may only show up once there is an active eSIM and a physical nanoSIM. You can have more than one eSIM configured but only one will be active at a time so dual SIM always means one active eSIM and a nanoSIM.

You will be able to associate things like whether the Personal Hot Spot uses your primary or secondary number, tied to the eSIM or nanoSIM, depending on how you label them.

I would just be happy for instanced if I can toggle the T-Mobile eSIM on and off and the changes are instant, so that if there are any problems in dual SIM mode, I can just toggle the eSIM on and off as needed.

Otherwise I normally use the nanoSIM for prepaid mobile data on my iPad, which is more useful to serve as the hotspot while maintaining my roaming US account on the iPhone. But in this case, the iPhone 12 has a 5G radio and there are actually 5G prepaid offers in some countries.