I'm not too much of a script buff, but this seems fairly possible using a event... Something along the lines of
If (ACTOR A) increases health
- increase (ACTOR B) health
ELSE
-
If (ACTOR A) loses health
- decrease (ACTOR B) health
Or somethin'... But, as of the moment, that means either making a common event script so that you can link to it in all battles and/or items, or finding some other way to automate this...
However, I haven't really looked at my RMVX in making that little dummy event, so I'm not sure if it will be even possible... I'll look into it.
(Sorry for necro-ing, I just really wanted to see this... Resolved, and now that I think about it, I think that I might be able to, kinda)