They drew an artificial line in the Pacific. Navy was in charge (Nimitz) on one side of the line, Army (MacArthur) on the other side.
I had to look it up - MacArthur had the Southwest Pacific - Australia, South China Sea, New Guinea, the Philippines, most of Indonesia (Dutch East Indies), Bismark Archipelago and most of the Solomons. Nimitz had the rest of the Pacific.
I think, but am not sure, that Guadalcanal fell on Nimitz's side of the line even though it was in the Solomons. - Found the answer to that - the line ran between the Santa Cruz Islands (Nimitz) and Tugali (MacArthur) so the line was shifted 60 miles west to give the whole operation to Nimitz.
Additional background on the rivalry here
http://www.dtic.mil/dtic/tr/fulltext/u2/a528913.pdf
(sorry, cut and paste).
Interesting. I thought Macarthur had it all but more or less left the Navy autonomous since he didn’t really care about them so much.