|I missed the following the first time through:|
not necessarily at the same time
If you're not looking to run them at the same time then you needn't worry about having the two LAN's on different subnets and you can put the default gateway address on the streaming interface. But, if you do ever want to run them concurrently, you will need to have the two networks on different subnets or you'll really freak your computer out and it likely won't communicate with anything at all....so keep that in mind.
I presently have 3 interfaces on my work PC. One connects to my main client network. The second I use for configuring switches at my desk, and the third connects to my management network (in case I need to connect to a network appliance at a remote site).
I simply leave the two I'm not using disabled at all times. When I need to switch, I disable the active one and enable the one I wish to use. When I'm finished, I disable it and re-enable the one I need at that point. As long as you always disable the active NIC first, you won't run into trouble.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.