Thank you FIS
I can't believe it was there all the time!
So, apparently the flip flop is the a change in master FMGC where no vertical mode is engaged nor A/THR mode displayed in the FMA (but is A/THR active?).
To solve it, engage V/S.
What about disengaging and engagin again?
Anyway, there is no point for a pilot to engage them simultaneously during teh cockpit prep doing a tai chi like movement.
I have another question for you:
if we engage FD1 and then FD2 within 180 ms, we'll we have a flop-flip instead?
haha just kidding