looked around but couldn't locate an answer. Can two or more investigators fight the same monster and team up together to kill it?

No.

Remember that while each turn of the game is divided into 5 phases, each investigator has his own "individual turn" within each phase. The next investigator doesn't move until the previous investigator is done with his movement and combat with monsters usually happens during movement. The same goes for Arkham Encounters and Other World Encounters as well.