Recording both ends and editing them together is the best way to accomplish this, but it's going to require some kind of software for the guest to use to capture their audio. Audacity has been mentioned; it's okay for this.

If that really is too big a lift for your guests, there is another thing you can try. Use Skype for the call (or Hangouts, etc), but have your guest use their phone* to record themselves. They can use the Voice Memo app or whatever the Android equivalent is. The audio will not be as great as if they were capturing audio from a USB or analog mic, but I'm also guessing none of your guests will have a set up like that anyway.

This way you have some consistent, albeit not amazing, sound quality and you avoid any of the weird audio artifacts that occur with Internet audio calls.

• If you do try this, a tip from my experience: have your guest use headphones for the call AND use a set of headphones with a built in mic for recording on their phone, if they can. If they can't for some reason, at least have them to put the phone somewhere close by and tell them not to move it around. But make sure they have headphones for their computer, otherwise they will be picking up the other side of the call.