Progress report on my attempt to go bigger.
I originally purchase the 500mm Shapeoko2 set. Then ordered the 1000mm rails. But I really wanted to use full sheets of plywood with their 4ft widths. I noticed that if I could combine the old 500mm with the 1000mm then a I could fit the 4ft sheets and by sliding the sheet through I could theoretically cut on a full 4x8 sheet. I have heard that such a long axis may become prone to going off square but so far after a few projects the machine is holding up great.
If anyone is curious. I combine the two makerslides by tapping both top holes that are in the middle of the makerslides. Then put a piece of all-tread (screw with head cut off) in holes and screwed the two pieces together. This took some wiggling so that the slides lined up pefectly vertical and tight together. To keep them from unscrewing I found a double T-slot nut to go into the bottom track. For holding the two makerslides parallel and avoid twisting in the axis 3 bolts were used with spacers in-between the slides.