Why can't I host concurrent meetings with two pro account licenses?

HI, Ksdc.

A pro account user can only host one meeting at a time per device, and a license is per user, so I would first check if the licenses are assigned. You should be able to host a meeting with user A on one device, and user B on another device.

These documents might be of help: 

https://support.zoom.us/hc/en-us/articles/115004976063-Assigning-a-License 

https://support.zoom.us/hc/en-us/articles/206122046-Can-I-host-concurrent-meetings- 

Hope this information helps!