Actually, the STS can handle two MOH channels. I've never done this, but the manual says:

"This feature also allows the system to assign CO line circuits as additional music inputs. This increases the capacity of music channels beyond the one available on this system. A total of seven channels are available for use on the system."

We've also had success with the STS. The hard drive based in-skin would be your best bet for two companies.

Gainesville Telephone Systems
Jacksonville Telephone Systems