I will likely have two domain names pointing to the same IP address. I might need to spread the load out between two servers. So, I might want domainname_1.com to point to server_1 behind my router and domainname_2.com to point to server_2 behind my router.

My understanding is this will take two steps:
1) Set up my router to forward port, say, 81 to server_2
2) Set up domainname_2.com in dynu.com's system to redirect to xxx.xxx.xxx.xxx:81

Is that correct? And will the user see the ":81" that gets added to the request, or will the ":81" stay behind the scenes?

Thanks