Bone Simple K2YWE SO2R Audio Box
My limited experience as an SO2R absolute novice caused me to conclude that the simplest solution is the best one for me. That meant only switching receive audio. I always use headphones, which made the desired functionality even simpler.
My solution uses two DPDT switches and no power. The choices are Radio 1, Radio 2, or both. The two radios are isolated in different ears in the "Both" position. No mixing is performed. One switch selects Radio 1 or Radio 2. The other switch selects either the Single Radio or Both Radios mode. Here's the schematic.
I oriented the Radio 1-2 switch bat handle horizontally and the Single-Both switch handle vertically. See the sketch. You may want to change the arrangement to suit your own preferences. In my case, the radios are side by side and the Radio selector handle points to the desired rig. The other switch points up for Both.
I suggest you do some pre-wiring if you use a small box where wiring might be tight. First, carefully site the components so as not to interfere with each other. Be sure you allow room for the phone plug tip, as well as for rotating the jack so that all lugs will be accessible for wiring. Then make all the holes in the enclosure and dry fit the components in their correct places. Remove the parts when you are happy with the arrangement.
Now mount only the switches in their correct holes, but mount them "inside out" on the outside of the box. This provides a jig for pre-wiring. Keep in mind that the switches will be upside-down when you wire them, and you will have to flip them over for in-box installation. Make all the inter-switch wiring and external cable connections with the switches externally mounted. Put flying leads on the Single/Both switch poles for later connection to the phone jack. Cut the external cables to final or longer length. Do not install the phone plugs yet.
Remove the wired-up switches and reinstall them in their proper places inside the box. Run the loose ends of the cables through their respective holes in whatever order works best for you, last, first, or along with the switch installation. Tie knots in the cables or use some other means of strain relief. Ty-wraps work for that purpose.
Install the phone jack. Cut and strip the flying leads from the switches to length if you have not already done so. Solder the flying leads to the phone jack along with the appropriate wires from the external cables. Use the sleeve lug on the phone jack as a common return (ground) point for the cables. Finally, attach the phone plugs to the ends of the cables. You're done!
Plug your headphones into the box and the cables into the transceiver headphone jacks. You should hear the selected receiver normally in the "Single" position. When "Both" is selected, you should hear a different receiver in each ear. The box is wired to connect the left output from #1 to the left ear of the headphone and the left output from #2 to the right ear in the "Both" position. This assures hearing the main receivers of Yaesu rigs during "Both" operation even when Dual Receive is selected on an individual transceiver. This may also be the case for other brands.
No relays, no active components, not pots. This project is a simple point of departure for your own Mods. You might want to add a third pole to the switches for moving your keyer output to the selected rig (you'll have to decide what both means in that context!).