You're right, you WOULD get two halves when you cut the tubes in half--it's actually a cool idea, though executing it neatly would take a lot of work. If you shaped the pipes accurately where they butt together you could join them with the same pvc cement plumbers use for dwv (drain-wast-vent) piping. Might work! If you used 4 inch pipe, the bike frame might fit loosely enough in the trough that it would accept most frames of a given size (60 cm or whatever) even if the geometry isn't identical. And most of us are only going to be cleaning bikes in our own size, right? It would actually be something like making a big fat-tubed plastic frame and slicing it in half...

When is someone going to weigh in on the electrolytic method? I tacked it onto this post because I figured that people interested in OA might also be interested in electrolysis. I posted on this separately months ago, and haven't found any knowledge or interest. Hmm.
JV