Is there a reason that you don't update the out of date ADNM server?
I haven't run into this configuration yet, but I'd assume that the best course of action would be to have all versions up to date.
The only downside that I can think of is that the ADNM console of the out of date server might not allow the newest updates to be pushed out (unlikely), have missing features that the newer version has, or might be vulnerable in some way.