Maybe use a converter that fits both the output and the input.

What adapter? As far as I know, all Sonos adapters are either RCA or 3.5mm jacks. 

Or something like this, perhaps?

Those are certainly banana clips, as I mentioned in my reply here.

I’m using these.   Very hard to get 2 wires in one hole.  
 

What size wire are you using?

So you’re connecting speaker wire to a Sonos Amp?

Yes. I’m wiring 4 Sonos speakers to an Amp.  Using this wire;

14/2 Speaker Wire 14 Gauge (Copper Clad Aluminum) - Fire Safety in Wall Rated Audio Speaker Wire Cable / CL2 Rated / 2 Conductors - CCA

Maybe use a 2-way speaker selector with impedance protection/matching, or if that’s not preferred, then go for ‘stackable’ 4mm banana plugs instead. Try Amazon, or other reputable sites online.

These were suggested, but out of stock.  I’m looking for something similar.  

Ocelot Banana Plugs, 24k Gold Plated Connectors, Open Screw Type, 12 Pair, 12 Pair (24 plugs)

I’ve not used any to suggest an alternative to you, but maybe others here who have, will chime in.

Using a spare section of wire, practice cleanly removing the insulation without fraying the wire. Don’t twist the wires, just slide them straight in. I wouldn’t expect any trouble with two #14 wires.

Great suggestion!  I’ll try that!  Thanks!