Backing a wood doesn't make it less brittle... It might prevent the back from giving up, but the belly could still collapse.
"Mahogany" (either Sapeli, Sipo, genuine mahogany or African mahogany) is one of those woods that just don't like to be bent. How many bows have you seen of any of these species? I recall none. Weird, since the wood is very common as lumber. This says something to me... The wood just doesn't like bending. A hickory backing will probably overpower the mahogany very easily. If you need to use mahogany, I'd use a thin, lighter backing such as Ash, or maple. Trap the backing and keep the belly flat. Fades at least 2" wide, with an overall bowlength longer than otherwise needed.