I am not too sure, for it was a long time ago but I may have seen Nick Drake play one song with John Martyn. This was when he was doing a guest spot at Les Cousins, on John's night. It would have been only one song as he certainly did usually play alone. Sorry if I gave the wrong impression. by alongside, I meant on the same bill.
I think that when their artists die, it may possibly make life easier and more profitable for their managers and producers.