The first Kripke structure satisfies both S1 and S2:
0 1 2 3 3 ...
[b SU a] : 0 1 1 1 1 ...
F[b SU a] : 1 1 1 1 1 ...
Fb : 1 1 1 0 0 ...
[Fb SU a] : 1 1 1 1 1 ...
For the second structure, we have the following
0 1 2 2 ...
[b SU a] : 0 1 1 1 ...
F[b SU a] : 1 1 1 1 ...
Fb : 1 1 1 1 ...
[Fb SU a] : 1 1 1 1 ...