Well so far I have been told from my Buddies at Mini airboat that "Center" is about 1/4 1/3 of the length
From the back. Cool.
NOW ?
I have 2 seats one in each "Cat" there a little forward center boat length.
Do I have to put sandbags in the seats to simulate people, so it "balances" in the proper place ?

Well, here's the thing...If you look at most airboats the operators seat is typically around that magic area where it will balance out even with an operator in the seat...Knowing that your seats are forward of that optimum area in my opinion I'd say yes add the bags then install the accessories to help balance the boat..

Your build will be even more affected by the weight distribution and those twin hulls...Too much weight up front and you'll have a hard time steering...

Thanks
All things to work out