Hello, and welcome to JustAnswer.com!My name is XXXXX, XXX I'm here to help.
Phone numbers are managed and assigned to SIM cards. She can't have more than one phone number on a SIM card, but she can use Multiple SIM cards with one phone. She'll have to switch them out manually, but she can do that whenever she wants.
That should do it. If what I recommended doesn't help, or if you have additional questions, please let me know and we can continue troubleshooting.