Working on the js files at the same time will result in lost work as "the last one in will win" and overwrite any of the other changes made by the other dev. There really isn't a good way to handle this within Kentico aside from using CI or your own source control repo and allowing the changes to be merged into each other from there. Also implementing CI isn't simply checking a box and saying "lets do it", it involves quite a bit of change on your dev end.