Not everyone may know this, and it may not be obvious. Once you have a .Win account on one sub, you can use the same account on all .Win subs.
If you have an account on one .Win sub, just log-in with the same log-in info on another .Win sub. You don't have to make an account more than once.
I know it is weird that (as far I know) you are not automatically logged-in on other .Win subs when logged-in on one of them. In a sense they are basically websites of their own while also being able to share accounts some how.
EDIT: Ofcourse you should make sure you only do this with Official .Win subs, which you can find a list of here.