Subnet Bridging?

HOW DO U CONFIGURE SUBNET BRIDGING!!!
I Have 2 NETWORKS at work, First one is responsible for Internet Acess & Network Neighbourhood & Second one for R & D.
ie- Both have separate pool of IP ADD;
e.g: #1-Network has 209.249.204.xxx, #2-Networkhas 192.008.162.XXX.
Can I use a single NIC by using a switch for the 2 Networks or I have to use 2 NICS…..(I WANT THE USERS TO HAVE ACESS TO BOTH THE NETWORK AT THE SAME TIME)
Can anyone help me A.S.A.P–THANX IN ADVANCE
( I AM USING NT 4.0 FOR WKSTATION & PLANNING TO CHANGE THE OS ON THE SERVERS FROM WNT 4.0 TO SOLARIS 8.0..)