Once you have hooked two speakers up to one channel the load presented to the amp will no longer be 6 ohm.
If you connect in a
series the load will be 12ohm, if you connect in parallel the load will be 3 ohm. If your amp is 4 ohm stable you will most likely be able to get away with it. I wouldn't go jumping on the volume knob though. You would be safe with 12 ohm.