There is no problem having two different sized batteries in a single bank. They just need to be the same type - AGM, wet cell, or gel cell.
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.