Nope easy as

ok so it works best as a layer cake (just 2 cakes joined with buttercream in the middle)

so just lightly grease 2 x 20cm cake tins (one at a time if you only have 1)

bake for 35mins until a skewer comes out clean.

If you don't want to do it as a layer cake just bake 2 cakes :-) - you can freeze the uniced cake...
Does the ingredients in the first post make two 20cm cakes? Or do you have to do a double batch?

I was thinking of halving the cake mix batch and icing it on top? So I would halve the recipe you gave to make one 20cm cake right?

am I making any sense?