Think DC is right & you would be far better beefing up the almost non existent gantry. You can't change the Y axis to supported rail without affecting the Z axis as the bearings at present are in the same block as a major part of the z axis. Not sure a double gantry is the way to go & probably at least as much work as sorting this gantry properly. Don't have the experience to comment on the double gantry but guessing the idea came from the machine that was discussed on here not so long ago.