its possible - I hosted 2 servers using the same install of BHD, and played on both with the same install on a third. As long as you don't log into play the cd key isn't an issue.. which I never do.
(in fact, I currently use 1 cd key to run 4 servers (3 of them at 1 time and on 1 computer) and of course have it installed for playing).
It is going to depend on how you setup hosting:
1. Lan play
if the server is set to host on LAN, then on the other computer when you go to play make sure to select LAN and the server should show up.
2. Internet play
If you set the server to host through novaworld so that others can come in and play with you,
then what you need to do is get pinger and manually add the server to the list, using the internal IP address of the computer running the server.
Then when going to go play multiplayer, they just click your server on the favorites list and they should be good to go.
At least that is how I have to do it.. even opening the correct ports, if I try to join my server hosted on the same lan as me playing, i have to connect directly to it via pinger