I've had double FMGC failure a couple of times. Once near top of climb which was a bit of a non-event and the other at high speed on the takeoff roll. The first time, we had to reset the FMGCs manually but the second time they eventually rebooted themselves. Both were a while ago but from memory the first signs of trouble were resynching messages on the MCDU followed by loss of FDs, A/T, A/P, characteristic speeds. For the takeoff roll case, there was some guidance on a "F/D Off" takeoff which is essentially similar to the double FMGC failure.