You didn't make it clear whether you were going to leave the internal speaker connected (Assuming you do)

OPTION A (Not recommended)

Connect the extension cabinets together and then use one output on the amp to run both 1/(1/8+1/8)= 4 ohms and this is way under the impedence the amp expects. Plugging in the internal speaker as well makes things worse but only slightly 1(1/16+1/4) = ~3.2 ohms. You could also just plug an extension cab into each of the available outputs for the same (4 ohm) result.

The main pro here is you only use normal cables and don't have to rewire anything. The con is a pretty serious impedence mismatch.

OPTION B (Recommended)

Make up a special speaker lead that connects the two cabs in series to each other and use this in the external speaker jack with or with out the internal speaker connected. 8ohms + 8ohms = 16 ohms.

Pro - Doing it right
Con - Special lead that is required

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