I am new to this and need some input please!! I am making 2 12x18 sheet cakes and icing them as one cake to make a 18x24 full sheet cake and stacking a 12x18 on top then on top of that will be a large mustache shaped cake carved from a 12x18. Each layer will be 2 inches with no torting. Do I need to place cake boards and dowel the top 2 layers? It will be BC icing and not sure yet but maybe cover the mustache with fondant and make fondant hair or BC and grass tip. If you have any suggestions on what would look better please, please let me know! I love this site! I have learned so much just browsing around.

I'd dowel the bottom two layers... the top layer (the carved mustache) doesn't need supports if it isn't supporting anything.

It isn't necessary to dowel every 2", but the full sheet, half sheet, and carved half sheet probably aren't going to be cut all the way through for serving, so I'd use a board under each layer in order for them to be served typical 2x2x2 inch slices.

The basic rule of thumb is to dowel anything that goes over 4" or 5" tall.
I like what CWR said about using boards to make it easier to serve.