It really comes down to how 'connected' do you want or need the second office to be to your Headquarters.
How much sharing of files/emails/services will there be between offices?
I think the only way for domain controllers to be connected is through a VPN, but you might be able to get some WAN / VPN optimizers that compress and QoS the traffic to minimise your bandwidth constraints...
Alternatively, you could just set up a totally separate domain, and either use a POP3/IMAP plug-in for your emails, and perhaps Terminal Services for File Share and Admin.
I'm sure you'll get some good input from the other TZ members too. Goodluck.
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