Ok I sort of answered some of my questions but still have a few.
in case of dual latch, we are directed to Select the NAV B/UP prompt on both MCDU DATA pages.
(Refer to DSC-22_20-60-130 General HOW TO USE, concerning navigation backup
operation)
During Dual latch: -22_20-90-10
1) Both FMGCs are inoperative. FM and FG capability are lost.
2) AP/FD, A/THR are lost
When referring to 22-2-60-130, one can see this caution note :The MCDU NAV B/UP is to be used only in case of FM 1 + 2 failure. It can be selected temporarily in case of FM1 or 2 only failure, in order to ensure that the function is available on the failed side.
When in MCDU NAV B/UP on both sides, one FG at least must be available to engage AP and A/THR.
So, section -22_20-90-10 tells us that both FMGC's are lost including FG.
Yet, 22-2-60-130 tells us at least one FG must be available to engage AP and AT
I am confused.