I am having trouble finding an adapter, which I wouldn't think would have to be very large, that has two 1/8" male jacks, would sum each of them to mono, then send each one to a different channel of a 1/8" female jack. Any ideas?
Sounds like a custom-build job.
Let's call one 3.5mm plug "LEFT" since it is going to the left ear. We'll call the other one "RIGHT."
Solder the TIP and RING of the LEFT plug (male) to the TIP of the 3.5mm jack (female). Solder the TIP and RING of the RIGHT plug to the RING of the jack. SOLDER the SLEEVE of both the LEFT and RIGHT plugs to the SLEEVE of the jack.
Bob's you uncle.
You need to be a bit more specific in your question. Are these stereo plugs that you're converting to mono plugs, or do you need mono signal through stereo plugs on both ends?
Are you talking about starting off with two 1/8" stereo male plugs and converting each plug into an 1/8" mono male plug?
You just need to build a cable that goes from 1/8" TRS to 1/8" TS. Solder sleeve-to-sleeve, and then solder tip & ring both to tip.
Or are you talking about starting off with two 1/8" stereo male plugs, converting each plug into a mono signal, but still ending with a 1/8" stereo plug on the other side?
In that case, it's a bit more tricky.