How do you run multiple servers or worlds at the same time with MineOS Crux?

Well, I found the answer but I can't take credit for it. I emailed the creator of MineOS Crux directly, William Dizon, and he got back to me in the same day! Very impressed.

Turns out that the default installation of MineOS Crux sets up the Linux firewall (iptables) to only accept incoming Minecraft connections on the default port, 25565. That means even though my second server was merrily running away, any attempts at connecting were blocked. Mr. Dizon passed on this link from the MineOS Crux wiki that explains how to add additional ports to the iptables:

https://minecraft.codeemo.com/mineoswiki/index.php?title=Iptables

Once I followed the steps, set up my additional port for the second server, everything worked! I had forgotten about iptables, and that is a little embarrassing, but live and learn... again ;)

Thanks again to Will Dizon!