- Suggestion #1 - Please don't
Suggestion #2 - If you ignore suggestion #1 please be careful and don't get mad when things go badly.
Companies like Riverbed exist, and it's taken a lot of effort at Autodesk to develop Revit Server, because data integrity (Bits and Bytes/data transfer/latency) between Central and Local files (element permissions) is not trivial. Drop Box (I really like it btw) is really just copying files from one place to many places. It does it quickly but not quick enough for concurrent activity...not all the time, every time, never fail. It does nothing to manage Revit permissions and that's what will burn you, borrowing something at the same time as another person. It comes down to what sort of gambler you are. Comfortable with losing 10,20...30 minutes works?
There is hope...IF you are extremely competent with Revit...AND...the kind of person whose pen and pencils are arranged in a very specific order on your desk and can tell if someone touched your work area in some barely perceptible way...AND...the other person(s) you are going to try this with is your twin in this way...you might be able to pull it off. Might help if you are going about it as if you are playing Halo (WoW or COD) online and communicating (via a headset) with the other person continuously as you work.
It works great to share a project file if you and your teammate are "following the sun", you start working on it when they've finished for the day. The real danger is concurrency, doing stuff at the same time and the very real risk of borrowing the same thing at the same time.
I know there are people that have done this and have had success. There are exceptions to every "rule". There is a thread at RevitForum.org and post this morning at The Revit Kid advocating it works. What I'm most concerned about is people diving into a real project and having a go, then dealing with hours of work that can't be reconciled because they weren't prepared enough for the worst.
As I wrote at the beginning, if you ignore #1, don't be upset with #2. You've been warned. Be careful out there!