This question might be because my head doesn't function very well in the morning but if you have 2 offices connected via VPN, can you use just 1 small business server? Why would you have a SBS setup for each office?
They can either access Exchange across the VPN or with Outlook's "RDP over HTTPS" Internet protocol. I't's not painful at all. Millions of people use hosted Exchange (or other remote mail servers).And with SBS that also means users at one site will be accessing their Exchange mailbox over the VPN which will likely be painful.