If the mean time to repair is 0 then you cannot recover.  However,
if there are not two simultaneous failures, then you can always recover.

P1 = A ^ B ^ C
P2 = A ^ B ^ D

A fails:
A = P1 ^ B ^ C
P1 becomes A ^ B ^ C ^ D
resue P2 as A:
P2 = A

[time passes]

B fails
B = P1 ^ A ^ B ^ C
reuse P1 as B
P1 = B.

no more failures can be tolerated.