Q: SIM from phone in iPad

Depends on the SIM ...

iPhone and iPad: Understanding which SIM card your iPhone or iPad us

Why do you need to do that? Turn on your Personal Hotspot and connect your iPad to your iPhone over WiFi for Internet access.

iOS: Understanding Personal Hotspot

Hmmmmm. I think so, but I'm not sure. I haven't tried it. hihihi

Ok, but what is the charge? Is it worth the time you will spend switching back and forth? Also, your operator may still be able to tell that you are not using the same device. In this case, Carolyn's advice is good. First check that it's possible with your SIM and then ask your operator.