The port that SolarWinds listens on is the Default Port of UDP2055, but you can change that port in the SolarWinds Settings Page - This way you can send the NetFlow for SolarWinds on a different port. When you say "another NTA Server" do you mean a different instance? If so then it can listen on any port you define as it's on a different IP Address and you send "SEND" Netflow from the end device to SolarWinds. SolarWinds is just a listener.
David is correct in that Netflow can send on any port you want, and that NTA can receive the Netflow on different ports just fine also.
And its fine to have another NTA server, whether it be Solarwinds NTA or a different companies NTA, receiving Netflow on a different server. However, if you wanted to have 2 NTA servers, you'd need to integrate them into 2 different instances of NPM. You can't have two NTA servers both working with the same NPM instance, nor can you reliably have 2 NTA servers both writing to the same database.
Just to expand on what cnorborg has said - if you were to deploy additional polling engines into your environment, each Polling Engine acts as a NTA Listener/Colletor - So you could, in theory, send your NetFlow to different locations based on geography. Of course, it would all eventually be sent into the NTA Database for collation, but be warned NTA doesn't deduplicate.
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