The best, and safest, way is use a router. Other than that, XP, and newer versions of Windows allows you to bridge multiple network devices. I think that if you right click on Network Neighborhood and go to properties, and then right click on one of the devices and go to properties, you have an option to bridge the connection. This was really designed for laptops with a wired and wireless device, but that should work. Be forwarned, this will allow users from ip1's network to have complete access to everything ip2's network, either way you do this, unless you firewall between them.

The best, and safest, way is use a router. Other than that, XP, and newer versions of Windows allows you to bridge multiple network devices. I think that if you right click on Network Neighborhood and go to properties, and then right click on one of the devices and go to properties, you have an option to bridge the connection. This was really designed for laptops with a wired and wireless device, but that should work. Be forwarned, this will allow users from ip1's network to have complete access to everything ip2's network, either way you do this, unless you firewall between them.