ESim and Physical Sim in Iphone 12

Yes, you certainly can.

It’s a GREAT feature of all iPhones Xs/Xr and newer.

Note the caveats in article below that the device must be carrier-unlocked to use different carriers AND that the device will only support a SINGLE CDMA network.

Using Dual SIM with an eSIM - Apple Support

Note also that while your “12” can have multiple eSIMSs installed, only ONE eSIM can be actively used at a time.

What is it you are REALLY trying to ask ???

You’ve not defined what YOU might consider a “problem.”

Example: You can’t “call yourself” (from one of your lines to the other).

However, MANY users - including me - use two lines routinely and EXACTLY as described at:

Using Dual SIM with an eSIM - Apple Support

You’re welcome and yes, it really DOES work.

Just read the linked article linked above carefully.

As for installing your new eSIM, this article should be helpful:

About eSIM on iPhone - Apple Support

It really is dependent on the hardware configuration used by the individual carriers …

… but in all likelihood you’ll find it works GREAT.

Pragmatically speaking, finding more than one CDMA network in any geographic area is likely quite rare.

Yes !!!

(as long as they are NOT both CDMA networks)