I would suggest that you trim the dome off both layers with the serrated knife.
Put frosting on top the lower layer and then put the trimmed top of the other
layer next to the frosting. That way you will have a nice smooth flat top to
frost rather than a domed top. If you make crumbs of some of what you cut off
and mix it with frosting and "plaster" your cake you will fill in any space you
have between layers. Let it set for a few minutes to firm up and then frost
the cake all over with your frosting. A good way to smooth your frosting is to
let it firm up some on the cake and then take a wooden pizza roller, dip it in
cornstarch and roll it around your cake. This will get out many small
imperfections. Oleta