How to combine two disks

It already is combined - BUT xraid provides single redundancy, so the new disk is providing that instead of additional storage.  If you want additional storage, the capacity rule is "sum the disks and subtract the largest".  Another constraint is that you need to add 4 TB or larger disks to the array.

 

So you could get a second 4 TB drive.  If you replaced the existing 1 TB one, you'd get 4 TB of storage, if you simply add it to one of your empty slots you'll end up with 5 TB.

 

Another option (if you don't want redundancy) is to change to flexraid, and destroy the current volume (and the data on it).  Then you can create two new volumes - one for each disk.  Recreate the shares (some on each volume) and restore the data from backup. 

 

Note you can create a single volume that spans both disks, but I don't recommend that.  It's quite fragile - if either disk fails, you lose all the data on both.  Using two volumes ensures that only the data on the failed drive is lost.