One of you would have to change the port.
Ex:
Server 1
LAN IP: 192.168.x.101
Ports: 28000-28032
Server 2
LAN IP: 192.168.x.102
Ports: 28033-28064
Port setting in Blockland can be found in Advanced Config when starting a server.
Then port forward for both of those IPs for those ports.