Time machine on two remote computers

I'm trying to set up Time Macine on two Macbooks, using one external hard drive. One is a work laptop and one is a personal laptop. They do not share the same network. How do I get access privileges for what's backed up on my work computer when I'm on my home computer? It won't even give me the opportunity to put in my password, it just says I do not have privileges. Help! Thank you!