I have had this failure in flight, but not a dual channel failure, only single channel.
The indications are confusing, and there is as discussed no solace in the QRH. Everything will seem normal until rotation, at which point everything described below flags up. Your FDs will disappear as the aircraft clicks into flight mode. LNAV track is still available but no vertical information and you end up having fun testing your raw data skills your company is so keen on killing.
We ended up in a fun hour flying radar patterns to burn fuel before returning to our airport of departure.