How To Merge Two Codes when Develops are working on same con

Question

i am developing an application in mvc in my organization which is smaller firm now problem is that two developers are working on same controller on diffrent methods but when we merge two diffrent develops code we take it from one develops and by pendrive or email as medium and merge it to others develops backup and then make final backup. here i want to know when two develops are working on same controller or same module on bigger companies or mnc's then how to merge and make one final copy is they use any third party application for that or any other way please help me