hmmm, leaving them separate could let one of them shoot out from under your tire if you accidently hit the gas or brake too hard. It's bad when my whole metal ramp shoots out from under the tire on slick concrete.
Back to your question, brucepick. As long as I can remember, my dad always had a pair of ramps/stands made from ~14" pieces of 2x4 assembled log-cabin style - about the size of a milk crate. The top pair of 2x's was set perpendicular to the tires so the tires nestled between the 2x's and didn't roll. I don't recall his ramp configuration, but it was removable (say, that sounds handy).
Boy, we used those blocks to support all kinds of stuff besides cars, we stood on them, etc. I may need to reinvent a set of those
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