it's not at all odd to do this--to stack two 12" cakes up with a board in between to make a taller tier-- it's done all the time
fruitcake is normally a single solid cake, thats what people who are used to fruit cakes, expect.
I bake 4" high fruit cakes all the time, it bakes longer and I cover the top with foil after a while to prevent the top from going too brown, but it comes out perfecty every time.
Also keep in mind that the fruit cake will have a layer of fondant and one of marzipan. If your sponge cakes will only have a layer of fondant, remember to compensate for the marzipan thickness. I once did a cake where the fruitcake came out taller that the rest because of this. No-one else really noticed, but I knew :)
The alternative is to do a double board to get the look right.