Sorry for not getting these things earlier.
So in the custom hal I added two objects.
or2 and and2.
then I did
or2.0.in0 = probe A
and2.0.in0 = motion.digital-out-00
and2.0.in1 = probe B
or2.0.in1 = and2.0.out
motion.probe-input = or2.o.out
In other words, if P64 P0 is on then the and gate enables probe B. P65 P0 will then disable it again.
It seems to work but around long enough to know that I've missed something.
ps. I see a custom.hal, does a custom.ini exist?