Hello,
What you are proposing (two 8 ohm cabs on the same side) will create a 4 ohm load, which may damage your unit. See the following link for more information on ohm loads and cabinet connections:
Cabinet Connection and Ohm F.A.Q.
http://line6.com/community/docs/DOC-4410
Video Tutorial: Line 6 Head and Cabinet Connections
http://line6.com/community/docs/DOC-1565
Please let us know if you need further assistance, or close out the ticket as “answered” if your situation has been resolved.