Assuming each hosted site corresponds to a different IP address, you can specify the binding IP in VisualSVN Server properties. In this way, VisualSVN Server will be listening on this specific IP and will not respond on requests for other addresses (i.e. hosted sites). In other words, 443 port will be available for other addresses.
To specify VisualSVN server binding address :
- use 'VisualSVN Server Manager'
- right-click on your server
- go to 'Network' tab
- in 'Server Binding' section, check 'These IP addresses' and add the right address
If your hosted sites and your VisualSVN Server are reachable via the same IP address, I think you will have to change VisualSVN listening port from 443 to another port number (in Network tab; dont forget to restart VisualSVN server). The only drawack is that you will have to specify explicitly the port number in URL's to reach the SubVersion Repositories login page : https://your-domain:NEW_PORT_NUMBER/svn/