If you ever share a local connection or computer with another player, you need to register as multi-accounts by emailing the handles to firstname.lastname@example.org. As registered multis you need to be in the same faction or one or both can be freelance. As a result, if you would rather not be bound by that restriction, it would be best just to avoid logging in from the same connection.
I'm not sure I understand your question. Basically, if you ever share a connection with another player, you need to register as multis. If you do not want to be bound by the faction restrictions, just don't log on from the same place as another Combine member.
If you feel that your situation is not covered by this, just e-mail the details to email@example.com and I will be able to help you out.