Networking 2 companies

Please advise on the best way of connecting two small business networks.

Network 1 is already set up using Windows 2003 Small Business Server, and serves 5 users within company 1, and one user in company 2 (the 2 businesses are physically side-by-side).

A new network is to be set up inside company 2 with its own server – platform is not yet known but it is likely to be a Win XP peer to peer network, or client/server based on Windows 2000/2003 Server.

We want the administrator of Network 1 to be able to access all resources of Network 2. But the administrator and users of Network 2 must only be able to access specified resources on Network 1 (network 1 hosts DSL internet).

Please advise how we can achieve this. We can provide further explanations if needed.