Does this looks promising?
Nope, you never know what the parasitic diodes will do. You may be able to "test" this (how I detest that design method) and next time you get chips, they will be from another foundry and the parasitic diodes will have another effect.

The one sure fire way for one uC to know if the other is powered up is as follows (of course, the pins can be changed):
a p0.0 connected to b p0.6
a p0.1 connected to b p0.2
b programmed to set p0.7 to the state it sees on p0.6

a checking if b is there and powered
a pull p0.0 low and wait a while, then if p0.1 not low b is missing.
then
a set p0.0 high and wait a while, then if p0.1 not high b is missing.

equivalent for the other direction.

Erik