Ya. In stereo mode you could run a cable from each amp on the head and use all the power.If you mean the XL series HArtke's with the stereo/mono switch and two input jacks - no, they don't cut off half the speakers to achieve 8 ohm and add the other(s)s for 4. Obviously you could have 2 speakers on each "side", just use 8 ohm speakers ran in series on the magenta and blue wires for a 4 speaker cab. In mono position the right hand jack can be used with another 8 ohm cab to achieve 4 ohms total impedance.
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Electrically it's the same as running 2 cabinets. They just happen to be sitting in the same box.