Larger ok, but I wouldn't tandem two waste tanks
However, there are flush water conservation techniques that can increase by at least 50%--or even double--the number of flushes your existing tank can hold.Most boat owners don't realize that any manual marine toilet that's working anywhere near factory specs can move bowl contents at least 6' in the dry mode...they think they have to use the wet mode to push the bowl contents to the tank...filling up the tank with flush water. Here's a better way:Pump a couple of times in the wet mode to wet the bowl or add water as needed ahead of solids with a cup or the shower head...switch to dry...and after use, flush the bowl contents to the tank--or, if there's a vented loop in the head discharge line, enough to get over the top of the loop...at which point gravity gets it the rest of the way to the tank. Switch back to wet and pump only enough times to rinse the bowl...then switch back to dry to push the rinse water through the system. Not only will doing it this way reduce the amount of flush water going into the tank to a quart or so...it the added advantage of rinsing the waste out of the head discharge line, helping to prevent it from becoming permeated with odor.Try doing it this way for a while...you may discover you don't need a larger tank after all.