I am shooting with an unmodified Nikon D600 and want to take a shot at the Eagle Nebula. I have a clip L-Pro and a 2" UHC that screws in my reducer. Bortle 6 from a backyard just south of the Phoenix metro area.
So, should or can I use both together? If not both, which?
IIUC the UHC lets in a subset of the wavelengths of the L-pro, so you wouldn't get any benefit from combining the two, and would probably lose a bit of transmission.
If I were in your shoes, I'd just stick to the UHC. For the eagle, that sounds like the best bet.
With an unmodified Nikon, your H-alpha sensitivity may be extremely low. Inspect your red channel after you get a preliminar stack to see how it looks, the UHC should produce rather clean red data. but I suspect you may need several hours of it to get decent results due to the lower sensitivity.