I can't imagine the Seagull Harrier not rolling fast enough for you. What you are trying to do is a gimmick with no usefull performance gain. The elevator is on the far end of the horizontal rolling axis and will not improve roll rate much if any. Ask anyone who has lost one elevator half of a split elevator setup.

Now a usefull elevator mix with flaperons is possible. It can make hovering easier if your elevator response is sluggish while hovering.