Lots of keyboards give you easy fast access to the volumes of 2 sounds. In the MX price range, you could look at the Roland Juno DS which has dedicated sliders for the two sounds in a split or layer.
Another option would be to pan your bass to one side, and pan your top sound to the other side, and put them into a mixer, to get separate sliders or knobs for the two sounds. I don't remember whether you can pan sounds from the limied on-board editing included on the MX, but if not, you can do it with an editor. John Melas' Performance Editor should do it, for 30 euros. You can download a free demo. He also has a pack with other useful tools.
www.jmelas.gr