Ah, sorry, I misread the question.
I think the nearest for the second “them” will be English “those”. So, y rheina/y or y pethau 'na/hynny. Not sure about the people version of them, you are right, it would sound odd without more info. Might need different word order or something to fill in or explain before the 2nd nhw.
I think (but I’m not sure - happy to be corrected) that for “They left them behind” using the short form of gadael you could say “Gadawon nhw hwy ar ôl” - hwy is another way of saying them, and two 'nhw’s together sound wrong to me.
To be honest, personally I’d avoid the short form here and use the auxiliary instead - Nathon nhw .adael nhw ar ôl
Hi both. I did look up the word ‘hwy’ as ‘nhw nhw’ just sounds wrong. Also the word ‘ill’ if there were 2 or more people but that seems to only work if you know how many there are. E.g ill tri.
Using rheina makes sense as does avoiding the short form altogether. The only problem there is I’d probably have started the short form already before realising it didn’t sound right!
You will definitely hear people doubling up on pronouns in normal speech, though - gadawon nhw nhw would be pretty clear, even if it wouldn’t win any stylistic prizes…