So I have three users that belong to two companies. The companies are
connected in a business sense, but not in an IT sense. They are at
different physical sites. Both sites has an SBS 2008 server.

The goal is for the users to see each others calendars.

Company A:
SBS 2008
Exchange 2007 SP2
external url: mail.companyA.com.au

Company B:
SBS 2008
Exchange 2007 SP3
external url: remote.companyB.com.au

User1, physically located at Company B:
Outlook 2010 SP1
Company A login: user1-abc.lastname
Company A email: user1-abc.lastname@companyA.com.au
Company B login: user1abc.lastname
Company B email: user1abc.lastname@companyB.com.au

User2, physically located at Company B:
Outlook 2010 SP1
Company A login: user2.lastname
Company A email: user2.lastname@companyA.com.au
Company B login: user2.lastname
Company B email: user2.lastname@companyB.com.au

User3, physically located at Company A:
Outlook 2010 SP1
Company A login: user3.lastname
Company A email: user3.lastname@companyA.com.au
Company B login: user3.lastname
Company B email: user3.lastname@companyB.com.au

All have Calendar Read permissions on each other.

In order for each other to see each other's Exchange accounts,
Outlook Anywhere has been used for the distant site's Exchange account
with Basic Authentication.

User1 has got both accounts setup as
separate Exchange accounts via the Control Panel/Mail icon (same
profile) and can see them both as different accounts, and there are
separate OSTs (confirmed). As such, she has no problems seeing User3's
calendar.

User2 has got both accounts setup as separate Exchange accounts
via the Control Panel/Mail icon (same profile) and though I set them
both up with separate OSTs, for some reason, during the initial setup,
the 2nd account decides to use the 1st's account's OST. So it appears in
Outlook like there is only one account, the CompanyB account.
Fortunately, this doesn't seem to impact her ability to view User3's
calendar.

However User3, setup identically to both of the above, also
merges the OSTs (as described for User2) BUT it doesn't show the
CompanyA account (her primary account) but the CompanyB account, which
is no good. Then the Calendar shows the CompanyA calendar and does not
show the CompanyB address book, so she cannot choose User1 or User2 to
view their calendar.

Please note, that in case it was because the
logins were identical, I did change the logins to be non identical and
the same behaviour occurred.

So, can anyone advise me on what is going on here?

My preference is to have 2 separate OSTs in each Outlook, which I think will resolve the issue partially.
And how can User3 view User1 & User2's calendar

User 1 connects Exchange A via Outlook anywhere, connects Exchange B via RPC/MAPI.

User 2 do the same thing as User 1 but use the same OST? (Please explain how you do that.)

User 3 connect Exchange A via MAPI/RPC, connects Exchange C via Outlook anywhere. Use the same OST as User 2.

Is it right?

We recommand that you separate OSTs and then configure the Availability Service for Cross-Forest Topologies in Exchange site:

http://technet.microsoft.com/en-us/library/bb125182(v=exchg.150).aspx

Tony Chen
TechNet Community Support

Thanks for answering, Tony.

As I stated above, I did not CAUSE the OSTs to merge, and in fact would prefer that they did not. When I have gone back into the settings and configured the accounts to use different OSTs, Outlook ignores the configuration and reverts back to using a merged OST. I do not want this behaviour, and in fact this is the behavior that I think is causing the problem. I want THIS corrected.

Before I configure this "Cross-Forest Topology in Exchange" - can you confirm that I can create Cross-Forests between two SBS servers?