Normally, printservers that have several output ports, drive them by using different IP-Ports.
I have not found your specific printserver, but by experience most manufacturers do such things mostly the same way. Here is a "cut and paste" from hp's print server documentation:
•9100 TCP port is used for printing. Port numbers 9101 and 9102 are for parallel ports 2 and 3 on the three-port HP Jetdirect external print servers.
Hence you might try the port 9101 for the second printer.