I bought a 2-input Behringer UMC202HD interface with the hopes I could use it to play along with stuff I play on Spotify, Youtube lessons, etc. Seems the inputs are each one channel, so I plug into one, right ear only, plug into 2 left ear only. I assume this is by design since it's not stereo, just one channel like an amp. I installed Reaper, but just started so I'm clueless. Can I use reaper to make the guitar signal come out of both L,R channels? Can I mix Spotify in somehow too?

 

I don't use Reaper or a Behringer interface, but generally in all DAWs this would be how to quickly address this issue in your DAW:

1. Add a new *MONO* audio track/channel.
2. In the newly-created track, set the input to whichever physical channel you've plugged your guitar/vox etc into.
3. After that, you can then simply set that channel's pan to taste...

Note the above approach will necessarily introduce some small latency, depending on your hardware & interface settings. On the plus side you now have the option to add 'insert' or 'send' effects to your guitar/vox, to aid inspiration as you play along!