Hi there -- we have a few older answers on this, mainly focusing on why the USMC was not used in the European theatre. Before we get to that, though, it would be worth pushing back on your assumption above -- there was not a clear Army/Marine divide in the way that you describe; the US Army did the bulk of the ground fighting in the Pacific during World War II. There are a few reasons for that -- the Marines were meant to be expeditionary troops with a focus on amphibious landings, who were mostly stationed in the Pacific before Pearl Harbor. The Marines only had six total divisions by the end of the war, while the Army had 90 -- not counting the Army Air Force (the USAF as a separate branch didn't exist until 1948). The garrison in the Philippines, much of the ground forces in the Hawaiian archipelago, and the forces sent to assist Australia and New Zealand were Army units. While some Marine advisers assisted in planning invasions in Europe, the units themselves stayed in the Pacific.
To get to those older links:
https://www.reddit.com/r/AskHistorians/comments/1g1s56/why_didnt_the_marines_lead_the_dday_assault/
https://www.reddit.com/r/AskHistorians/comments/m02i52/i_read_earlier_that_in_ww2_us_marines_never/