Does an SQL Server instance allow both secure and non-secure connections at the same time?

I suspect that this article will help you out:

  • Understanding SSL Support

The protocol still uses TCP port 1433, by default, irrespective of SSL being used. The client can request SSL and, if the server has a certificate installed, the server and client will negotiate SSL. The server can be configured to force SSL, which will cause clients that do not support SSL to have their connections terminated. It is not necessary to tell the server to force SSL to get encryption, so long as the client requests SSL.