The only thing I want to add is that, event if you assign your multiple servers a unique port number, the iptables must accept data to pass through. That is one of the security measures Linux takes to prevent “backdoor” access. This is true no matter where the request comes from, internal network or internet.
This thread may help in getting that configured as well.
It took me a few tries to get my server working for multiple services, not just Minecraft.