Shared data server+2 Domains

Looking for a bit of advice please ..

I have 2 separate 2003 servers (DCs)on 2 (small) separate networks in the same building, but sharing an ADSL internet connections (via extra NICs and a switch/NAT etc.

I now want to put an extra data server on line which can be accessed from either network but still keep the networks separate.

Any ideas on the simplest way

Thanks in advance

Meldrew