Dual Companies

We are going to be working in tandem with another IT business. Is there a way to run two separate companies linked to two separate Quickbooks files? We'll be working together but would like to keep the companies separate.

If you refer to operation, e.g. managing tickets etc. then you can use a single system with more users and simply label/categorize accounts, tickets, etc. in a way that it'll be easy to tell which business should handle this. Though do note that it will not be a complete separation but rather an easy way to distinguish between the different records.
From billing perspective, however, you will only be integrating with a single accounting system, so in case you must integrate billing too you'll need a separate system, to which you'll need a separate license to. In case you need more details on this please do not hesitate to email us directly. Hope this helps.

is there a way to run/add more than one Quickbooks company file?

You can integrate with one QuickBooks Company, so that's why in case a complete billing separation is required you'll need to license the 2nd system.