Those VLAN interface IP addresses will be the default gateway for each respective VLAN. Same for the other company, only pointing to the other edge device.

I would think that you would want to setup 2 LANs (using vLANs). Use DHCP reservations to lock the company property into the correct internal vLAN.

We explain what is a VLAN (Virtual Local Area Network), How to configure them, and what are the primary benefits of using VLANs.