Welcome to Cake Central. There is an article about assembling a stacked cake in the articles section.
Using dowels to support the stack is important. There are ways to prevent the icing from sticking to the layer above: use extra powdered sugar in the center of the cake between the dowels, or shredded coconut can be used (watch for allergies)
I prefer the two to partially stick together, it adds stability to the tiers and helps prevent slipping. You are going to cut it anyway, so it doesn't really matter a lot.
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