Colonoscopy with EGD

Patient had a colonoscopy and an EGD during the same session.

I am billng 45378 1st, do I need to add 51 modifier to 43239.

I've done some research on this, and I have read "to add it" and "to not add it".

I need some direction... as well as guidelines as to why or why not.

Colonoscopy with EGD