2 Domains on 1 physical network

I have a customer who would like to share an office with another business. They both have existing domains, one running Server 2k3R2, the other running SBS 2k3.
They would like to both use the same physical network to share internet costs, etc.
They have no intentions of sharing data between the networks, so there is no need for any domain trusts.
Is SBS going to create an issue in this situation?