The developers can both work on the same dashboard simultaneously but only if they divide the work so that they work on different objects and maintain rather strict process on merging the results (which would need to happen quite often to minimise the possibility of errors made in that phase). They can use individual copies of the main .qvw for development and copy-paste their results back to the main file, but it might get tedious and will be error-prone.
It is much easier if they could work on different parts of the .qwv as Sunil Jain explained.
You can use SVN. Multiple developers can work locally and then commit the application,which updates the application residing on the SVN server.So the application on the SVN server is always the latest and you can also get the latest updated application from the server.
Hope this helps.