This is actually fairly common in close-up dialogue scenes and interviews. Even if more than one camera is available it may not be possible to get the angle you want without the second camera being in shot.
The way it is done is that each scene is shot twice (at least) each time from a different angle usually with both actors delivering their full dialog. This both keeps the performance natural and correctly timed and gives a full audio track with both sets of lines for a given take. The two shots are then edited together.
Bear in mind also that the audio can be edited separately from the video so you can have a line from take 1 continue over a video cut to take 2.
Having scenes edited together from various different takes is a normal part of video production and actors, directors and editors are used to dealing with it.
If a shot is from one character's perspective, ie. 862
an actor is talking directly to the camera, (to another character not the audience,) then this will be the only way to do it and it will inevitably by a bit unnatural for the actor but delivering a convincing performance in this sort of situation is part of their job.
Afterthought
Single vs multiple cameras is much more about a style of filming rather than the literal number of cameras.
For example a sitcom which is filmed 'live' is much more likely to use a 3 sided set, similar to a theatre stage, possibly with a live audience . With moving cameras used to film an entire scene in a single take similar to a live TV broadcast. This tends to work well for long running series as it is quick and cheap to film as you can reasonably expect scenes to be done in one or two takes and there is much editing required, just cutting between camera angles. For long running series like sit-coms this can also produce more relaxed and natural performances especially when actors are very familiar with their characters.
A single camera approach gives a bit more creative flexibility as its easier to use bigger, versatile sets and locations and it gives the editor and director more flexibility to create changes of pace and tone. 862