I am going to point you to this wiki page: https://wiki.visionaire-tracker.net/wiki/Team_files
Now according to the page, you can work on it at the same time, but I highly recommend you work on your project one at a time. It's easy for one person or the other to accidentally edit something the other person might be working on thus overwriting what they were doing.
Anyway, give that wiki page a read. If you want more information on the subject, then SimonS would be the person to ask / wait for a response for as it's not a feature I personally use myself.
One thing I will recommend though is that you backup your ved files regularly - never work entirely in a single ved file for the entire project as it's possible for them to corrupt while saving, so always make sure you create backup copies.