That is what I did with 3 of my old beds. It lasts forever, of course,
but here are some disadvantages. First, I did not bury the bottom, but
you should. That way grass rhizomes will not be able to push in and
invade the beds. I suggest burying them 3 inches.
Second, I was going to use no-till in my garden, and it works very
well, except that clearing radicchios and their immense taproots takes
a lot of digging. With sides that are not so rigid, they bend outward a
bit more every year. Third, when I come in with my wheelbarrow full of
manure and I dump it into a bed I will occasionally bend a side. The
last two problems can be minimized with double runs, as you say, and
also by planting stakes on the outside. I used old piping to make the
stakes. They help, but I did not put enough of them, and of course they
catch the hose all too often. All in all these are fairly minor
problems, go ahead and reuse that siding.