basically yes.
I'm logged in (as the same user) on my mobile phone with conversations and on my desktop with gajim. in this constellation i receive all in and outgoing msges on both devices.
( and i did never change the setting "route.all-ressources", and if I look at my config this key / value is not present here.)
I think this is related to XEP-0280.
Also I haven't tried two gajim clients... at least I'm not 100% sure i did.
Oh, forgot to mention:
This also works, at least parts of it, with pidgin:
When I've got a conversation with somebody, lets say on my desktop, all the in and outgoing msges get synced to my mobile device.
BUT:
When I switch to my mobile device and continue to write from there my sent msges don't get synced to my desktop. Received msges however are on both devices.
