ADFS 2.0 - 2 domains, How to setup User Profile sync to partner domain

Setup is;

No trust between the domains

Domain A (a.com)
SharePoint 2010 Installed here
User Profile Service configured for A Users
ADFS 2.0 Server Installed and configured (SharePoint is Relying Party, b.com ADFS is Trusted Claim Provider)

Domain B (b.com)
Partner Organisation
ADFS 2.0 Server installed and configured (a.com ADFS server is Relying Party)

All is working well, users from b.com can login to https://sharepoint.a.com.

Next - User Profiles - I can't get the sync to work for b.com users
- Forest name = forest.b.com
- Auth Prov Type = Trusted Claims Provider Authentication
- Auth Provider Instance = b.com ADFS
- Account Name = b.com\UPSsync
- Password = ******
- Port = 339

It won't populate the containers. I can't find any guidance on this anywhere! Do you need to have a domain trust in place for it to work?

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Paul Griffiths - MCTS, MCITP SharePoint 2010