I had that in mind as well; plenty of regulators on the market with a single pressure gauge so I thought, single gauge/one stage; two gauges/doubke stage, but then I was pointed that dual/two stage regs have something in the back of the body that differentiates them.
I bought a Matheson two_stage from a guy at the flea market for $50
Then I met another person selling what seemed to me good regulators byt they are single stage; he had only one dual-stage, another Matheson and some fittings for it but this one was a bit more $$.