I had this set up sort of as follows.
Connections from this notebook:
LAN >> Intranet
--- < OR > ---
LAN >> WAN
And also:
Wireless (Built in) >> WAN
The intranet is effectively fully disconnected from the web, but connected to each other via that left over modem/router.
I'm on wireless most of the time, but I like to go wired for certain things. With the above, I could have WAN from either wired or wireless, or I could have WAN from wireless and intranet from LAN, or some other combination.
Problem was, I had to physically unplug/plug the LAN cable to change which I wanted to connect to.
I was trying to figure out how to do away with that step. Somehow, I would need either a switch of some kind I could flip, or another IP port/address.
Then I remembered I had a few old USB >> wireless LAN dongles from years past. I found one that is rated at 150M and plugged that into a USB hub already in place. Once I got the IPs figured out, I now have:
Wireless 1 (Built in) >> WAN
Wireless 2 (USB) >> Intranet
LAN >> Can be plugged into either at will.
IPs are all set to Static, so I have three assigned to this laptop.