User Sync - is it two way or one way?

Using the UserSync plugin.

I have the ability for someone to register on Site A or Site B. I want both to be 100% synced.

I have it working such that:
Registration on Site A generates user on Site B

How to I set it up so that a registration on Site B generates user on Site A?

Is it possible to do automatically? If not, is it possible to do via a manual sync?