I'm adding to this topic because stringy has asked me about how I record my violin duets--

@stringy

In order to put to parts together, they have to have the same tempo.  Most often, I put the sheet music into musescore, and use that as a click track (just like the group projects); I use a bluetooth ear bud (I just use one) so that I can listen to one or both parts while I play and record the parts. 

On a few occasions, I’ve just played and recorded the 1^st part, and then I listened to that recorded 1^st part with ear bud while I played and recorded the 2^nd part.  If you play a lot of things by memory (I don’t), this could work well.  Regardless, you have to match the tempo as if you were playing beside someone.

Once I’ve recorded both parts, I copy the recorded files onto my PC & use video editing software—I use Movavi Video Editor Plus.  I know that there are other ones out there, and I’m sure they work similarly. In this software, you can see both the video & audio for your recording.  In order to put the parts together, you have to align the sound waves appropriately.  I've included a screen capture of an example below.

I’ve got the vertical red line on the beginning of the sound waves.  The software enables you to move/cut clips so that you can align things like this.  In this case, both parts start at the same time; it’s more complicated when they start at different times, but you get the idea—

Video Duet - Edit Software Example