If I leave the receiver setting at -12b, and change to bi-amp, both low and high still dissipate 10W, but now each amp is only "delivering" 10W.
Yes-ish, with the caveat that the split would probably be more like 4/16 than 10/10. Systems designed for bi- or tri-amplification generally have much larger amps for the woofers.
In the end, if I had my sound meter measuring before and after, the sound level should be the same, right? And, I gained headroom on each amp.
Yes to the sound level, probably no to the headroom. There are a bunch of different ways an amplifier can "reach its limits" - output voltage, output current, temperature are the main ones. Most clipping happens when the amp reaches its voltage limit**, although shutdowns tend to happen as a result of current or temperature limits.
Since you don't have an electronic crossover before the amplifiers you're sending the same signal into both amps, and so the voltages will be the same and voltage limits will be hit at the same point as before.
If, however, the amp is "shutting down with 4 ohm load" as a result of current or overtemp limits, then the splitting of *current* could help a bit although this is where the 4/16 split bites you and you don't gain that much.
** it's actually not quite that simple because higher output currents tend to drag down the power supply voltage so you hit voltage clipping a bit sooner, but you get the idea...
