Hi, I've made cakes before but never more than 1 tier. I'm making 1 vanilla sponge and 1 chocolate sponge. It's the same recipe for both cakes but I'm just adding cocoa powder to the chocolate one so I'm guessing they will weight the same?

Can anyone tell me how to stack them please? It's for my sons birthday so I really don't want it to go wrong!

Size? Height? Will they be split and filled? What fillings, icing, etc?

Do you mean like tiered cakes with bottom layer being bigger than the top layer? Or do you mean that they are both the same size and you just are going to put them one on top of another? Are you going to cut each layer in two and put frosting in between them? 

They will be 9" and 6". Not sure of the height, I have 2 tins of each size that I was going to fill and stick together if that makes sense? I was planning on jam and buttercream for the vanilla and chocolate buttercream for the chocolate one. This is what I'm trying to recreate with minor adjustments to the decoration.

What fillings have you used?

I want to use chocolate buttercream for the chocolate one and jam and buttercream for the vanilla one

So are you just wondering how to stack the smaller cake on top of the larger cake?  You will need a few dowels (cut to just shy of the height of your larger cake) and a cake board that is 6".  Assemble and decorate the smaller cake on the 6" cake board.  I usually will cut down the cake board slightly so that it is just smaller then the cake.  This way when I cover the cake in fondant the cake board "disappears."  Insert dowels into larger cake (I would use 4-5 spaced out in a circle, just make sure that they are placed so that the top layer will be supported by them) for support.  Slather a bit of buttercream on top of the larger cake in the center and then carefully place smaller cake on top (cake board and all).

If the party is at your home and you don't need to travel with the cake, then you don't really need to drive a dowel through the entire cake.  But if you plan you travelling with the cake, then you will need to insert a long dowel (sharpen the end you insert so you can knock it through the cake board a bit easier) so that the top tier doesn't move.

@Kels93 ‍, there are dozens of tutorials on YouTube and baking bloggs on how to stack a cake.  Just Google "how to stack a tiered cake".  The only thing I would add to Freckles0829's suggestions is to be sure your bottom tier is as level as you can get it (I use a spirit level) and that your dowels (or large bubble tea straws) are exactly the same height.  Insert them into the cake as nearly perfectly vertical as possible.  A  level cake is far more stable than a slanted one.  Good luck!  

Thank you so much for your help! Fingers crossed it'll all go to plan!