If they're in series, using both amps, you're cutting the power they use in half or less because nothing is ground referenced anymore; you're just getting the difference between left and right channel. So it's not stereo, either: if you feed the same signal into both channels you won't get any sound at all.
If they're in series, using just one amp (which also isn't stereo, because you are only using half the sound information), seriesing from that channel's output to ground, you're probably cutting the power down to around 1/4 of what it was--first it's cut in half because you're only using one side of the amp, and then that is cut in half because you're doubling the resistance with two speakers in series.
Basically you're just reducing the total power used, so reducing voltage sag on the power supply.
Did you try testing as others suggested?
What were the results?
I notice that the repair to the battery you were using to power this amp here
https://endless-sphere.com/forums/viewt ... 2#p1339503
was never finished; so whatever problem you had there is probably still there and could be causing your present issue.
If you did fix that problem you didn't come back to that thread to report what it was and that it was fixed, so we have to assume it's still broken.