[primaeval]

Good its working. I thought I'd dealt with the ffmpeg overwriting issue. Obviously I was dreaming.

Newer Android versions have increasingly restrictive permissions on where you can write to which is probably why your second sd card is causing problems.
ffmpeg is also run in a limited shell within Kodi as the Kodi process's user.
I'm not sure if adding the card as Adopted Storage solves the problem. It becomes more trusted then.
Be careful: I think that might wipe the card if you change it to Adopted Storage.

You might have to put a different path in the "ffmpeg recording path" setting to deal with ffmpeg's point of view.
I don't know what that is though.

You might be able to change the permissions of a folder in the sdcard to the same as Kodi so it is writeable.
I expect you need a rooted phone.

Theoretically I could try to pipe the output of ffmpeg back into Kodi so it could write to the second sdcard but I got scared off doing that as some operating system specific issues cropped up.
It might have just been that it was troublesome within the binary IPTV Simple Client addon.
If I have the energy I'll have another look at it.

Another convoluted and probably tricky way is to find a network path that ffmpeg can write to, maybe an smb or nfs server running on the same device.