I plan on drilling and tapping the end of the outside bars. So you could effectively just put the next uprights there and chain them together, either so the bikes are the same way, or opposite ways, front or rear wheels. I've had my bike in there and it's pretty stable. Would be solid with two or more together.
My buddy has a CNC lathe at the shop, so if I could run the bars on there, it would go quick. I plasma the plates and get them powdercoated. I'm sure the more I get coated at once, the cheaper they would be. That would be the biggest cost.
FWIW, I bought that one ($40). It was cheaper on Amazon, but it was a complete *************. As you can tell, I build stuff, and I was about ready to throw that thing off of a cliff. Epitome of import junk. Holes didn't line up, bends weren't right, had to force things into place, crappy instructions, etc. The only thing that kept me from losing it was my 3 year old telling me he was proud of me and I was doing a good job.