From the hub PC, select the PC you wish to remove and click the link to remove it from within OneCare, manage your OneCare Circle.
Once it is removed, the PC will become unsubscribed and you can activate with the new LiveID for your second subscription.
If the activation dialog already contains a LiveID from a prior attempt or one used in trial mode, but this is not the one you need to activate with to associate it with your subscription, see this post for how to reset this - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/f5ce50e4-9165-40a5-891b-fe5d0659738b
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator