If the boxes are physically identical, you can use the dd command to duplicate the filesystem:
Assuming /dev/hda is your drive, add a second identical drive that will be /dev/hdb on a second controller for efficiency. It is best to boot the machine with a rescue CD or floppy so that the filesystem is not active when copied.
dd if=/dev/hda of=/dev/hdb
will copy everything on the original to the second drive.
Remove the second drive and install it in the second box. The two boxes will be clones.
If you use them together you need to give them different identities (hostnames,IPaddresses)
otherwise one can be a backup for the other.
If the boxes are not physically identical, it may be necessary to do a fresh install on the second machine. e.g. if the second machine has a boot device not supported by the kernel, it may not be possible to boot. In most cases the copy will be able to boot on a similar machine but will require some reconfiguration for X, for example.