Utility I'd like to see: SHARE. Command line version: SHARE [Src] [Dest] /[Write enable]. For non-Sparta DOSes,there would have to be a front end to identify the desired drive identities.
When connected to another Atari 8-bit, with Rx and Tx swapped, and Command / Interrupt swapped (ideally both ways), the system running SHARE acts as a disk drive for the second computer. It should be possible (though likely confusing) to have both computers running SHARE at the same time.
So, SHARE D3: D1: would mean that D3: on the computer running SHARE would be seen as D1: on the second computer.
I'd prefer write enable as an option in order to prevent accidental damage - since Ataris generally assume they control the disk completely, having two computers share a single drive could cause problems.
Ideally, the software would run in the background and be interrupt driven, so the computer running SHARE could still be used for other purposes when not serving data to the other computer.