What's the simplest way to combine two parallel signals at unity gain?

[ashcat_lt]

Does the inverting summing amp change this somehow,

Yes. It's fundamentally different in operation, as the mixing is done as non-interacting currents into a virtual ground.

This part just clicked this morning.  That inverting input is a ground, so the voltage divider I was seeing...well...isn't...because the "bottom resistor" is essentially shorted, or otherwise doesn't matter.  The voltage in question is actually that across the "top resistor", which is sort of a voltage divider with the actually source impedance, but it only really matters in context of what current that produces through that resistor...  It gets a little blurry at this point, but I think I suddenly understand it a lot better.  Thanks

[PRR]

> maybe best if all of those inputs are kind of close to the same impedance, but I'm not completely sure

Does not matter.

You can drive seven 1Megs and four 10K all from one source, as long as the source can drive the resulting 2,457 Ohms. (Most opamps can.)

In general, a Distribution Amp has a minimum load (per-output and/or total), but no maximum load and no need for matching loads.