I see what you mean, but synchronised should mean to me that the active mode (and submode) is the same in both, the master FMGC being the one determining which mode is active.
For instance, when you are flying a non GPS fitted 320, one ND can be do the left of the track, the other to the right, and the roll bars will give opposite commands to intercept the track. The FMGS will follow the master FMGC bar. But the mode is the same: NAV.