Two banks of two batteries - different size - no-no?

When I added a second battery I put in a switch that would allow separate and in parallel but planned only to use them individually to get around all the problems disgust so far. I also like the knowledge that when I switch I have used roughly half my capacity and can plan around that. I tend to use the older battery first and only switch to the newer one when we are off the grid long enough to need the extra capacity.