git annex sync supports git remote groups, that should be expanded to all
commands supporting --to and --from. And since there will be a [Remote]
list, could also support --to foo --to bar etc.

--[[Joey]]

Started working on this in multifromto branch.

Problem: Command.Move uses onlyActionOn, so only allows one remote to
process a key at a time. Seems that onlyActionOn needs to be expanded to
include the remote.

Problem: The concurrency is not right yet. For example,
`git-annex copy --to foo --to bar -J2` will start actions copying a
file to both remotes. Assume the copy to bar finishes first.
Then it will start an action copying the next file to foo, despite a
copy to foo already running. 

To fix this, it would help to look at Annex.activeremotes when starting the
next action. If foo is busy, start the action on bar instead. It would
be possible to build up a queue of actions that have not yet been started on
a remote. That risks using a lot of memory if one remote is very slow.
The queue would need to be capped at some amount, and when full,
delay until the laggard remotes catches up.

Bonus: git annex sync --content -J2 already works, but it has the same
problem described above and ought to be able to be fixed the same way.

Also worth noting that with --auto and -J, git-annex may make more transfers
than preferred content settings demand, because it will start several
transfers to different remotes at once. If only one copy is needed
amoung all the remotes, it won't notice and a copy will be sent to all
remotes. I think this is something the user can understand though?