What's the expected behavior when logging in from two different machines behind the same router as guest? I was expecting random user-ids for each user but I keep kicking myself out. By design?

Testing the webclient but I wouldn't expect that to make a difference.
Per Lummox JR's comment in this post, it looks like the computer_id (and thus the guest key) is based solely on IP for web client connections.

Connecting via Dream Seeker will work, because it can gather more information about the machine than a web browser will expose, allowing it to generate a unique computer_id for each machine.

Any reason you can't leave one of them logged in for testing?
No reason. Just didn't expect the webclient to use any identifying information. Don't see a need for preventing multiple guest logins if the game doesn't care.