I just received an email from Memphis about this.
Let's see; two M3122D subs wired in series-parallel would equal a 2 ohm total load- that's not enough power.
two M3122D subs wired in parallel would equal a 0.5 ohm total load- that's too low of an impedance for your amp to handle.
If you keep the M3122D sub you have, and series the coils (4 ohms), then take the M3124D sub with coils in parallel (2 ohms),
then put the two subs in parallel with one another, you'll get a total ohm load that is less than 2 ohms, and not only will there
not be ENOUGH power per sub, there will be twice as much power on one of them as there is the other. So that's really not
an option either.
All in all- your one M3122D sub with the coils in parallel (1 ohm) like you've got it now is your best possible scenario.
Is he just being safe or is it really better off to stay on one sub?