2 players on the same key? On different servers, I don't think it would work. You would keep getting disconnected. However, if you go on the main blockland site and navigate to the "Support" section, you will find
this.
Bascially, you can as long as they're using the same IP. So it could be in the house anywhere. Also, you can have some friends install the Blockland Demo and they can connect on your LAN server. But you have to be careful about giving your key out. As seen
Here.