How do you record a conversation between two people when one person is doing both of the voices?

Record all of each character's lines into a separate file for each. Then you can chop these up into clips, and insert them into multitrack, where you'll have a lot of flexibility with the placement of them. You can even get them to talk over each other if you want, just by sliding the clips around.

Yes, you use two tracks. In theory you could do it on one, but it gets confusing and there's absolutely no need. Using two tracks also means that you can use different effects for each voice if you want. There are any number of basic guides out there for learning about Multitrack, but most of them cover just about everything except what you want to do, it seems.

The easiest way to achieve what you want is probably to record each file in Multitrack view and save it, but then open the files and split them into clips in Waveform view. To do that, you find the section you want to use, highlight it and hit F8. This turns your selection into a marker range, which is displayed in the Marker panel, and you can give each of these ranges a name to help you identify each one. Now, if you right-click on one of these markers, you'll get an option to insert it into a Multitrack session. Ideally have one open first, and then you'll get that one showing up as an option. When all your clips are in there, you can then rearrange them as you want, and all you have to do when you've got the result you want is to either mix down or export the result.

That's the bare bones - you'll discover all sorts of things on the way, but that should get you going. There are some other possibilities, and it can get pretty complicated - for instance, read this thread...