I have a cake coming up that I am making for a friend. She picked out a two tier cake, top tier covered in fondant with a pearl boarder stacked onto a buttercream rosette cake.    

My question is how do I stack the fondant covered cake onto this bottom buttercream base?  

Is it best to cover the top cake in fondant and add the border made of pearls then stack onto the bottom without the roses on it yet? Then just pipe the roses on the bottom?

Any advise would be appreciated.

I would put the pearl border and the roses on site.  As always, make sure you put the dowels in the buttercream cake.