Ways to Manage 2 Development Projects on Same Section

Hello,

Now having a situations where there are 2 projects occurring, at the same time, on the same section of code in a webpage.  Looking for suggestions on how to manage each project so that project 1 doesn't overwrite code in project 2.  The projects are more complex then listed below.  The time to complete each project is usually due to slow movement from testing to production.

Simple Example:
Have an employee table and an employees info page.  Project 1: Format the phone number.  Project 2: Change the phone number from 1 column in the table to another column in the table.
Both would affect the same area of code in the web page.

Is there a standard way of managing this so the development doesn't overwrite each other when updated?  Any ideas are appreciated.  


Thanks
Jess