When you pull a given load from two dissimilar batteries, the effect upon each is different. As an example, if he pulls 100 amps out of a bank consisting of one 200 amp and one 100 amp battery, he lowers the available capacity of each differently - 50 amp from each but 25% of one and 50% of the other all else being equal.
That's the first problem.
The second problem created is in recharging both - the regulator or shore charge doesn't see each battery separately and starts pumping at a rate of the avg which will undercharge the larger battery and over-charge the smaller.
We are going to disagree on this one, but since it's not at the core of this thread I'll not go deeper here other than to say that the voltage remains the same in both batteries, so the percent discharge of each is also the same. Charging works the same way.