On the web based TV Anywhere you can log in using a secondary My VM account, that can be set to use a third party email address as the username.
How do people in my household use Virgin TV Anywhere?
However for the Sky websites only the primary address is accepted.
For the apps again only the primary details are accepted, but they are then remembered for quite a long time, so there isn't a constant need to know the password.
I know it's not the best experience but it's a combination of available technology and requirements from the content providers as to why it works the way it does.