Can I using my old number for an eSIM
Can I use my whole number for ESIM
iPad, iPadOS 15
An eSIM is simply an electronic representation of a physical SIM card. It is not a service that you can use simultaneously on a second device.
If your carrier allows it, and if it is truly what you desire, your carrier will invalidate the nano SIM card you are presently using and will move your telephone number to eSIM service. They will give you a credentials and instructions on how to install the eSIM service on your iPhone, after which you will be using the eSIM instead of the nano SIM.
It seems that this is not what you want.