I suppose if you just want to share files, all you need is some terminal software on each machine....
I'm not sure of the exact details, but I have used hyperterminal and a null modem cable I made to transfer stuff between some old PCs.
I think Freeze Dried Terminal was an ST program. I'm not sure of that was the exact name though....
I wouldn't know what Amiga terminal software there is, but you could probably find out on an Amiga message board.