I mentioned the Cokin filter system. It's a system of square filters, with holders and adaptor rings which really opened up the filter business in the early eighties. The adaptors are flat disks, with a hole and filter thread to fit the lens, and the outer diameter set by the holder. There were two sizes, 'A' and 'P'. Now, I've not tried this, but you can get plastic edging strip, a U-section, from various sources, and with the right sise it should be possible to fit two Cokin adaptors face-to-face, which would let you fix the two lenses together. Would it be strong enough? I'm not sure. Anyway, the Cokin system had conventional close-up lenses.