is it likely that the master/slave out pins on the back [i.e. Endorphins Cockpit] should be compatible with another manufacturer’s daisy chaining capable mixer module [i.e. Knob Farm Hyrlo]?

DON'T do this! Unless you know the exact pin-outs for both modules' expansion ports, you could be looking at an expensive, module-frying mistake! Consider: if the Cockpit or Hyrlo have any pins that send rail voltages (they're both powered modules), and they do so into an audio circuit, or do so in reversed polarity, or a few other potentially disastrous things, the resulting connection could literally blow up BOTH modules as well as doing potential damage to your power system and other modules in the process.

Only do this if both manufacturers say it's possible. But the best rule of thumb is to believe that anything going on BEHIND the panel aside of power and Eurorack-standard bus connections is going to be proprietary between manufacturers, and always presume that you can't expand Company A's module with one from Company B. Again, I'm sure there's exceptions...but find out what they are directly from the manufacturers AND find out if those connections might void your modules' warranties before even considering this.

sound logic and advice— thank you.

my logic in considering/inquiring was that,with only 2 pins per master and 2 pins per slave on the Cockpit, given it’s a stereo module, that must then be L and R channels of audio— no power voltage to consider. i believe the Hyrlo has 3 pins, so just murkier logic— 2 ch, of audio plus ground?

anyway/still: thank you for your quick help.

Even if you are not going to try anything potentially harmfull -- there is always the multi-meter to give some insight.

The Hyrlo probably has a hot left and right and cold common ground audio connection, yes. And sure, the Cockpit seems like it would be L+gnd and R+gnd...but what if it's not? Also, what if one module likes to see something like line-level at its expansion, but the other sends synth-level? Or if there's a big impedance mismatch? That'll sound quite awful.

Best thing you can do, unless you're working with either kit builds that you know the signal paths/levels on, or both companies say "yep, and this is how you do it...", just don't. Even if it doesn't result in catastrophic failure, it's still best left in the 'don't mess' category.