You find the loudest gain setting before the sub starts sounding shitty do the same for the other amp.
Now take a Digital Multimeter set to AC voltage to ensure each amp has the same output.
Disconnect the subs from the amps, unplug the RCA's going to your mids and tweets.
Play a 50 hz test tone on repeat, the stereo's volume should be at its maximum normal listening level that you use (not turned up to maximum).
Connect the sensor probes to the bridged channel of one of the amps, the DMM will show a voltage, now you want the second amp to have that exact same voltage output, so you pause the HU (to avoid shorting the speaker terminals) and connect the leads to the second amp, unpause the HU, a voltage will show on the second amp, ajust the second amps gain so that the speaker output voltage is the same as the first amp.
Now reconnect the subs to each amp and go for a little drive to listen to them, if they sound like they have too much or too little power just repeat the same steps above with a higher or lower voltage depending on what you want.
Note -with the RCA outputs on todays modern Head Units its rare to ever need to turn the amplifiers gain sensitivty higher than 50% of max, if you turn your gain sensitivty much higher then it needs to be you risk overpowering your speakers and damaging the voicecoils, and make the music from the amp distort as well.
You can use the cap if you want, it wont hurt anything, they are just not the magical things that people claim them to be.