Someone who is the first of two or more people to discover something(noun)
WordNet. Princeton University. 2010.
Use "co-discoverer" in a sentence
"I'm applying for Radiology-Imagenology residency in my country, and as a fan of Nikola Tesla, I think it's also important to mention his name as co-discoverer of X-rays."
"In 1979 Francis Crick, the co-discoverer of DNA, lamented the blunderbuss nature of existing technologies."
""This really is the first Goldilocks planet," said co-discoverer R. Paul Butler of the Carnegie Institution of Washington."