Hi

the 6" will be vanilla sponge and the 9" will be chocolate sponge. Can I put the top tier straight into the bottom as there are only two tiers or do I still need to stack and dowel? If so, how do I do it?

That was supposed to say straight ONTO the bottom tier!

You still need to dowel the bottom cake. I usually use between 5-6 dowels under a 6 inch cake. If you don't use dowels the 6 inch will sink into the cake below and won't be stand straight. Hope that helps!

Oh I forgot, you must also have a 6 inch cake board under the 6 inch cake. You can you stack a cake onto another cake without one.

I'm assuming each of your tiers are 4" tall then You do need to stack and dowel but all that is needed is to put the 6"cake on a 6" cake board, then dowel with 3 straws (yes, plastic drink straws).  5 or 6 is way too many.  A cake board the same size as the 'upper tier' is always needed.