You can select an outbound route based on the extension number of the phone, so if you’re willing to set up a couple of extensions on the phones, you could do it.
As @dicko points out, though, multi-tenant on Asterisk and FreePBX is a LOT of work and usually ends up not working completely right. Virtualization or adding another server (to have two PBX instances, one per client) would be the right course in the long-run (and save yourself hundreds of hours troubleshooting).