Equilibrium is based on thermodynamics, not kinetics. It is dangerous to use kinetic arguments to determine the equilibrium constant. It is not generally true that Keq = kf/kb (though it is true that at equilibrium the rates of the forward and reverse reactions are equal). Suppose in your case the forward rate = kf [A] and the reverse rate is kb[B ]. At equilibrium
kf/kb = [B ]/[A] =Keq.
Now if [B ]/[A] is constant, obviously [B ]2
is also constant, and we can consider this the equilibrium constant for 2A
2B (though I can't see why we would want to do so). But it is not equal to kf/kb. However you write the reaction equation, the rate law doesn't change.