DNS is not binding to a Port. You bind DNS to a IP <-> Name
You can have 2 different sites on 2 different ports without any issues.
So http://TestSite:90 and http://Testsite:80 will work perfectly fine.
As a general rule of thumb you should only bind one IP and Port combination to one webapplication at the same time (host header excluded - some additional steps need to be taken so the certificates are fine)
So in you need to assign no port when creating a new webapplication, since 80 is the default for non-https sites. You should not modify the bindings (in general) in IIS if you work with sharepoint. Just supply the port numbers needed when you create the webapplication.
Answer to the comments:
Don't worry about the "load" on a port. The only load you need to worry about is the saturation of your network link and it wont make a difference on which port it communicates.
So taking this into account, you should, as the simplest solution, consider using a host header in order to make the second site available on port 80.
There is also a slightly more complex solution, in which you implement a web application proxy and publish the internal port 90 URL to port 80. But this is some added complexity I would not suggest based on your questions. If you do that you also need to ensure that your alternate access mappings are configured correctly.
So I would strongly suggest to use the host header approach.