Well, I managed to figure this out for myself, though it's so complicated that I'm sure there must be an easier way.
In MS Paint it's intuitive - you just open a second copy of the app, open Image B, copy it, revert to the original copy of the app and paste. Then, after positioning the new selection you simply click elsewhere on the drawing to let go.
MT Paint is a different story.
I could find no way to open a second copy of the app for a long time.
Eventually, I stumbled across: File-Actions-Edit Clipboards to do so.
Then, having opened Image B in the second MT Paint, copying it was a real pain. I had to go: Selection - Select All, then Copy, then Edit-Save Clipboard. It took an age to find that combination by trial and error.
Then, switching back to the main app, I had to do: Edit-Load Clipboard. Having placed the new selection where I wanted it, it took another age to figure how to make it let go of the selection. Selection - None was the obvious choice, but that just made the selection vanish and I had to re-paste and reposition. Eventually, I discovered that with a rapid right-click then left-click, the app would let go of the selection and create a duplicate copy of the selection, which I could then get rid of by Selection-None.
What a rigmarole - and not a word of it in the 'manual'.
If anyone knows a simpler way, I'm all ears.