That's why every pilot knows (or ruddy well should) that "You've got her" (or similar) means you are to take control and that "I've got her" is the correct response, indicating he should now keep his paws off. In the event of an emergency where a pilot thinks he should take control unilaterally, he should announce the fact and ensure that an appropriate response is received (assuming his counterpart is conscious).
If both pilots are attempting to fly the same plane at the same time, there's been a catastrophic failure of basic procedures somewhere along the line. This sort of thing is usually referred to as "pilot error" in the inevitable post mortem.
There is one exception here and that is when for some reason (e.g. hydraulic failure) the pilot flying the plane is unable to move the control surfaces, at which time he may call for the bloke in the second seat to assist. Applying extra "grunt" in this manner is utterly pointless on a fly-by-wire aircraft and should never be used in such.
Thus if lack of synchronisation becomes an issue, the lads at the controls have fucked up. Big time. Fortunately the aviation industry isn't above telling pilots what they should damned well already know, so I'd expect such an alarm to be amongst the recommendations of the inquiry.