Why does the number of accessible microstates always increase?

If we consider two systems with number of accessible microstates A and B and internal energies of E(A) and E(B) that are exchanging an small amount of heat Q from A to B we get the following:

A total amount of access. MS before the heat exchange: W(1) = A * B

The MS of system A changes by a, MS of B changes by b.
The total amount of access. MS after the heat exchange:

W(2) = (A – a) * (B + b) ->
W(2) = W(1) + bA – aB – ab

The change of access. MS of the single systems are:

$$a = dA/dE = dA/Q$$

$$b = dB/dE = dB/Q$$

For a simple system where $a$ is equal to $b$ its easy to show that $W$ is max if $A = B$ (like maximizing the area of a rectangle at a fixed circumference)

However, since the number of access. MS of a single system increases exponentially with energy, and E(A) > E(B) -> a > b

How can we show that W(2) > W(1) // bA > (aB + ab), or probably related that W is in a maximum if A = B ?