Trying to join 2 clips , one after another

Hi, I’m very new to this forum.
I’m trying to join to 2 clips, each of about 20 secs, so total will be approx 40 secs.
The 1st one is mono and the 2nd is stereo. I’d imagine i’d have to change the stereo one to mono before i could join them ?

No need to convert.
Use the “Time Shift Tool” (Time Shift Tool - Audacity Manual) to arrange the tracks so that one starts after the other, like this (below). The tracks will be mixed together to produce a stereo file when you Export (File Menu: Export - Audacity Manual)

Excellent, many thanks Steve.
Will give that a go.
Kind regards, David.