I don't think I have explained myself very well.

when the machine flexes, its making the gantry 'lean' backwards Some of the flex is in the Y rails and some is in the X rails.

Doubling up the gantry would give me 4 X rails and the load would be spread out more evenly on the Y rails, and also the load wouldn't be in one direction but opposing so in theory will reduce flex.

Think of a balerena on her tip toes dancing on polysteyrene, she would likley just sink into it, but then think of a tank with huge tracks, it would likely glide over due to te increased surface area and balance over a larger area.

Hope I am making some sense?

On a side note, IF I decide to do this mod, I will be adding supported rails too in order to eliminate that possibility.