I'm still not exactly sure what you want to do with them. All I can say is you need to keep the small 18 ah battery s separated.
I would tend to put the lowest capacity batteries with the lowest output inverter if you are using it as a portable because the less draw the longer the batteries will last. If you want to run it at home or a fixed setup, You could put the the large batteries together and run both inverters off them and leave the 18 Ah battery's aside. That too is doable.
There is another possibility I have done often as has a few people I know.
Put a shipload of panels with a big controller on small batteries and run a big inverter off them. During the day, the panels will provide the power more or less direct and the batteries will just act like " Ballast" for the inverter. If you get a passing cloud or small interuprion to the panels the batterys will take up the lesser output from the panels and then basicaly recharge once the sun is out again. Obviously it pays to have more panels than the output you require as with the controller. I have run Multiple controllers to get the Amp handling up and I have also run panels direct to the battery when I knew the draw was going to balance it all out. If you have an over/ under voltage alarm which I bought a little battery charger board off fleabay for, you get an audible warning when you are going too far either way. For what I was doing I could run a good many hours of the day with this basic but powerful setup.
A lot of offgridders I have spoken to are going the same way. Panels, particularly used ones are cheap as chips now. I picked up a load of 250W's the other day for $10 ea and I don't even buy anything under 225's now. I can pick up 190's from the scrap yard and the guy is happy to give them to me. Size does not matter, it's capacity that counts. The offgridders are going BIG on panels ( and controllers) and smaller on the still expensive batterys. Only have to get them through the night.
On your setup if you had plenty of panels replacing the power used, and even 300W isn't much, You can get aaway with using the small batteries easily whilever the sun is out. Night time you are reliant on battery capacity but not through the day when the panels can virtualy provide straight through power. I ran a 1000W inverter a while back just for fun from 2000W of panels and a single 7.2 AH battery. The load pulled the panel output down from 30V and the thing sat happily under 15 V for hours.
You can couple the bigger batteries into one bank and use both inverters with all your panels or you can seperate the systems. Just don't put the little 18Ah batteries with the big ones and you can do any config you want. :0)