Interesting! I hope someone knows.
I've done that sort of thing, over the years. Bought cheap used filters just to get the mounts with traditional retaining rings. Recall some cheap filters are seemingly just bonded in there, must destroy the mount trying to get the glass out?
While we're at it, combining two absorption filters goes as expected. The transmitted spectrum is the simple scalar product of the two alone. But if either or (especially if) both are dielectric interference filters... the combination is most definitively not the scalar product of the two! I worked out the math and wrote a paper on that, long time ago. [The generalized mechanism is a combination of coherent summations over all past possible histories and incoherent summations between the two (or more) filters.] We then applied it to products (the customer was Xerox). Fascinating theory and immediate pragmatic applications... quantum mechanics in action!
Here's a couple of my cartoons and the (most important) summary, for limiting cases of the two-filter case. >>> Tom
Anyway, my point is... combining interference filters may give disappointing, even terrible results, manifesting as light-leakage, fringing, and degraded wavefront. Stuff like that. Tom