If the server will be off-line and only active as a fail-over in case something goes wrong, your best bet will be to image backup the entire server using commercial or freeware backup imaging software. This is usually accomplished by performing a bare metal backup, which provides a heightened reliability vs. a live backup.
Instead of re-installing the server on a second machine, you should be concerned about simply performing the backup operations. This will streamline the process and eliminate the need to constantly carry-over both files and Active Directory data. By performing an image backup and recovery to a second drive, or system, you are ensuring that you can use a second server as a test-bed. However, you must also take into consideration the End User License Agreement to ensure that you are able to run multiple licenses on multiple machines. You cannot, for a fact, run two SBS on the same Windows network at all. The SBS has to be at the top of the Active Directory forest and no other Small Business Server can be placed on the same network due to restrictions in place by the software itself.
This is because the SBS line of products has a reduced cost and feature set for small business environments.