Book in multiple series

How do people handle a book that is a cross-over between more than one series?

I've got 2 entries in my catalogue, one for each series, which seems a somewhat clunky way of doing it, but I don't see any other way given that both series are different and one is not a sub-series of the other.

For what it's worth, the book in question is The Energy Eaters by Arthur K Barnes and Henry Kuttner (it's on Gutenberg) and it features the lead characters from Barne's The Adventures of Gerry Carlyle and Henry Kuttner's Tony Quade.

It's slightly complicated because both authors wrote separately and in collaboration.