Put a new MX in the DC, in routed mode, with all of Company B's servers behind it, inside your existing firewall (you don't show this on your diagram, but I assume you have one?).
Put it in a different Organization from the existing Hub and existing Company A office Spokes (maybe configure a new Organization). Put the the Company B MX65W as a Spoke of this new Hub, in the same Organization. With the Company A and Company B MXs in different Organizations, the VPNs will be entirely separate. If the MX65W is already in the existing (Company A) Organization, you can logically move the hardware between the Organizations yourself, but you'll need to call Support to get them to move licensing.
If you are mixing your Company A and B servers on the same switching, this gets more complex, but you imply that's not the plan. If you keep it separate physically, it's harder to get wrong.