Delphi - work on the same project at the same time

Try to use  svn

http://docwiki.embarcadero.com/RADStudio/en/How_To_Use_Subversion_in_the_IDE

thanks

maosalah has the right idea, but id sounds like you might need a bit more verbose answer.  What you should be using is a "version control software" package.  SVN is one such package.

A version control software package allows you to check in your software to a central repository.  When developer A wants to get the source code he would get it from the repository.  This act places the source on his computer for him to work with.  The same goes for developer b (and as many more as needed).  Then, when one developer wants to make changes, he "checks out" the file he will be changing.  

Some version control packages lock the file so others cannot change it at the same time.  You can think of this as a library approach.  One developer checks out a book.  Others cannot check it out until the book is returned.

Others version control packages, like SVN, support more modes.

Either way, the source has been placed on each developers system so each developer can compile when they need to.  The following link compares 7 different version control packages.

http://www.smashingmagazine.com/2008/09/18/the-top-7-open-source-version-control-systems/

Let us know if you need more.

if it's a big project
you may want to split functionality too ...

the bigger the project, the easier it is to divide functionality into smaller programs and to maintain them

you can however keep a directory with common units for all the projects
but that requires that you make sure your code is in good order
like not using circular unit reference, good directory structure, etc

split functionality sample:
managers only rarely need to enter an invoice, but want to see reports
> write a reporting tool
>> with a way to assign reports to specific users

Hello,

I found a solution of my problem and it was much simpler than I thought.

Let's say we used Invoice.dpr. I created an invoice2.dpr, with exactly the same files so both of us can work.
I use Invoice.dpr, the other programmer uses invoice2.dpr so no Exe conflict.

That is going to cause confusion.  If you both make changes then how will you incorporate both?

Developer 1 will modify unit1
Developer 2 will modify unit2

The point is they can test the same project without waiting the other