To the best of my knowledge there can be as many servers as you want joined to an SBS network. The only caveat is that the SBS box has to be the Primary domain controller, and handling all the FSMO roles. HTH,
Yes, the SBS server needs to be the FSMO role holder and can't do domain trusts.
If the SBS box is just doing DC, DNS and DHCP, why not add the disk to the SBS server and use its backup facility to backup all the data? Or add the 2nd server as a DC, and also make it a file server and add some redundancy?
After everything that has happened during the month of Jan 07, I do believe that pigs fly backwards!