How to use 2 fins with the Starsurfer or Kode ?

[Sailboarder]

I bought a Starsurfer 117 this winter. It came with two large area plastic fins of about 31 cm and a normal 38 cm fin.

I'm suprised that the 2 large fins are of the same area. I thought it would be better to have a smaller fin at the rear when I use a large on in the middle.

I have two small girls, maybe 30-40 kg. They are using 2.0 and 2.5 m2 sails. I think the CLR (Center of lateral resistance ) with 2 large fins will be way too backward for their small sail. And if it is not, will they be able to rotate the board?

Should I trim one of the fins or try to find an elusive small Tuttle fin ?

Thanks!