I am about to have a new asphalt shingle roof put on my house. It now has
one asphalt shingle roof on it and the decking underneath is in good shape.

Other than it costing a little less by saving on the tear-off process, is
there any real difference between ending up with one roof or two?

Even if it costs more, if there is an advantage to having the present roof
taken off first, I would do that.

Thanks.