If you look closely at the DFF output behavior as you
go from hold to setup in a very fine stepping, you will
see the CK-Q delay stretch out, then a narrow plateau
where the pulse "stalls" in the middle, and then then
try and revert, and then show no capture of the intended
D value. Real "metastability" is where the FF never makes
it to a valid logic state, or fails to do so within the inter-
clock period.
The second DFF's regenerative gain will make a decision
based on level or noise, except with a very small window
of uncertainty. To fail you need the first FF to be "undecided"
(which is different than "uncertain", the basis of setup /
hold window limit setting) when the second FF's clock
arrives, and for that "indecision" to be at a level that
the second DFF can't resolve one way or the other (of
course getting the "right" answer is preferable, but wrong
or right are preferable to "umm...".
There's no such thing as 0% probability of a 2-stage
scheme failing metastable, but the statistics are squared
(odds of first stage metastable at clock2, times odds of the
metastable output voltage being unresolvable for DFF2).
Of course this is all "analog-y" and you may have to
"stay between the lines" and believe what library logic
models and rules tell you.