I've a really basic linguistic question (I think): what does one call
the situation where two speakers communicate each in their own tongue
but understand each other's speech? It's not codeswitching as I
understand the term, since each speaker is more or less consistently
using one tongue.

Over the years I often ran into situations where people would say that
they understood ("hear") another tongue, but couldn't speak it. I have
only rarely witnessed exchanges on this basis (at least where I could
identify that each conversants was pretty much consistently using
something different from the other), but read about it in the case of
Ndonga and Kwanyama in Namibia (these are very close, like dialects of
the same language, Oshiwambo).

TIA for any info.

Don Osborn