Suppose S1 knows the location of B.
This implies that B has sent a message to A, because B->S2->S1->A is the only path from A to B
Since S1 knows the location of B, it will not flood, however, by the above implication, we know that S2 will also not flood.
Therefore whenever S1 doesn't flood, S2 will not flood.
Therefore the given situation is impossible