To me, filling both sumps with, and using K1 and rings, is a bit redundant, because both media types accomplish the same thing, they are both bio-media, and more, is not necessarily better.
You do not get a larger bacterial population because you have more media. The biofilm/bacterial population is "not" based on volume of media, but based on the food supply (ammonia and nitrite) produced by your fish, and millions of beneficial bacteria can fit on the head of a pin.
Of course it is possible to have too little, if you overstock, but ......
In my 125 gal sump, filtering my 180 gal tank, I use 3 little, hand size bags of rings, and an 8"x 8 x 3" pumice block, and my ammonia and nitrite reading are always consistently 0.
The Porrett foam wall, adds a little extra biofilm space, but is really there for its mechanical filtration.
In my sump about 80% of filtration is done by live aquatic and terrestrial plants.
This is because once a tank is cycled, I consider nitrate the most important chronic toxin to filter out.
And by using plants,( indicated by my average test below), my sump does that.
And as you can see by the sump below.
About 10% That dark area on the left side), is bio, and mechanical filtration, and where the pump resides, the other 90% of the sump, is dedicated to the planted filtration part.
