How can a team of two collaborate?

My friend has the free version of unity, I have the pro version, but will not be using pro features until he gets the pro version as well. What is the easiest way we can collaborate and work on the project together? We were going to use dropbox, but I can see problems with that possibly. What other ways of doing this is there? Preferably the cheapest way possible, considering we are just two indie developers.

 

Search for unity with bitbucket + sourcetree (its free)

 

I don't need to pay anything? Is it error free as well? Because I would like to be able for both of us to work on the scene at the same time as well etc.

 

Can use the free plan: https://bitbucket.org/plans

Working with same scene is going to cause problems..

There are some plugins for scene merging, maybe somebody knows if there are useful?

Personally I would work on separate "work-scenes" and then just copy-paste what was added there, into the final or "combined" game scene.. (or take turns working on the scene)

 

What advantages does using Git have over Dropbox?

 

Long detailed write-up about it
http://forum.unity3d.com/threads/version-control-you-and-why-you-need-version-control.210791/