I know this is an old post, but I couldn't find anywhere else on the internet that had the answer.
^Strangeduck hit the nail on the head with his guess
When you AMM-1 strapped amps, you do it like normal. Run 1 wire through the machine, then put the clamps on both amp's positives (one amp should have the positive wires from the subs [master amp] and the other amp will have the negative wires from the subs [slave amp])
the AMM-1 will then measure impedance and power from both amps COMBINED.
source:
Two SQ2200s @ 1 ohm each
Juice box (34hz note the entire song) saw me rising to 5 ohms collectively (2.5 ish per amp) and clamped around 1500w