You can use Rebex assemblies with 2 separate licenses in a single application, as long as versions of those assemblies (DLLs) are the same. For example, you can use Rebex SFTP 7.0.8816 and Rebex SMTP 7.0.8816 in a single application, but you can't use Rebex SFTP 7.0.8816 and Rebex SMTP 7.0.8865.
As for setting this up - if you are referencing Rebex DLLs from installed packages, install the same version of both Rebex Secure Mail and Rebex SFTP, and reference DLLs that were installed.
If you are using NuGet packages, add "NuGet keys" of both licenses to Rebex.Licensing.Keys
collection instead of assigning a single key to Rebex.Licensing.Key
property:
Rebex.Licensing.Keys.Add("==FKkxI6bNJi....zAfy=="); // add key for Rebex SFTP
Rebex.Licensing.Keys.Add("==FcX7EDXoJF....CDRl=="); // add key for Rebex Secure Mail