I am planning to shoot the NEOWISE Comet
Assuming you mean one of the Canon EF 70-200mm lenses with a Canon EF 2X extender and an EF→RF adapter, yes you may.
But honestly, depending upon which EF 70-200mm lens (there have been seven different EF 70-200mm "L" lenses released between 1995 and 2018) and which version of the three 2X extenders that have been released over time in the EF system, you might have better results shooting with the bare lens and cropping.
If you are using a third party 70-200mm EF mount lens andor a third party 2X teleconverter, the waters get a bit murky. Older third party lenses and TCs might not work with newer Canon cameras without firmware updates. My Kenko C-AF 2X Teleplus Pro 300 DGX that was purchased in 2011 works well enough with with a 5D Mark II and 7D, but has issues with the newer 7D Mark II and 5D Mark III. It locks up the camera unless AFMA is totally disabled (turned off) in the camera's menu before attempting to use the TC.