Hi… This does work in basic cases, at least…
$ fly deploy -a some-app --image registry.fly.io/other-app:<long-string>
(Where long-string comes from fly m list -a other-app.)
If you’re deploying in an automated setting, then you do need a token that has access to both apps simultaneously, if I remember correctly.
(The new Macaroons system might allow you to fine-tune that more.)