FreeBSD, merge two hard drives

Of course for your purposes you would be better suited hardware (or prosoftware) stripe (RAID0) is not necessary once again to think about partitioning the disk, etc.
Well, in the absence of such, I recommend to use the following configuration (names for scsi devices and classic partition table):
/dev/da0s1a + /dev/da1s1a = gmirror partition for / (root) size for example 1 GB
/dev/da0s1b /dev/da1s1b = two independent partitions for swap (each the size of RAM in the server)
/dev/da0s1d +/dev/da1s1d = gstripe /tmp
/dev/da0s1e +/dev/da1s1e = gmirror /var
/dev/da0s1f +/dev/da1s1f = gmirror /usr
/dev/da0s1g +/dev/da1s1g = gstripe /home
\r
To supply the system with such a partition table, I recommend to use mfsBSD for the initial breakdown of disks (or installation system)
\r
The influence of the alternating sections with a mirror and a stripe on the overall performance of the disk subsystem has not been studied =)
\r
PS: For reactionaries there are still gvinum is almost LVM. However, I feel so gmirror and gstripe a lot easier to use

The simplest way to use mount_unionfs.
For example, if the first disk primary partitions, including /home, and the second one to add to the main /home, then mount the partition the second disk as /home2 and then do /sbin/mount_unionfs /home2 /home

Try geom_lvm.