Sure, if they have write permission to the folder. The problem comes when they try to file it; they could over write each other. Very dangerous problem.
The new software from ProE Wildfire allows multiple users on a model, but each user reliquishes control while the other one works on the model. The users are linked together. You can watch what the modifications are, but unless you have the control of the model you can't do anything but watch.
Try using Pro/INTRALINK. Multiple persons can work on the same assy members and assembly at the same time. Then everything is put together by combining their work.
I would not suggest working on the same part without any additional software control.
There may also be a problem if one designer models tha part and the other does the drawing. If the drafter makes for example a xsec (which is written in the part) there may be problems if the designer who models saves after the draftsman. Xsec will be lost. It is good to have a model locked (by Intralink for example) if you do a modeling.
Yes, multiple users can work on the same part at the same time if your using intralink. The function is called integrate and you can attempt to reconcile the differences between 2 models. OK now that you know you can, DON'T. Unless the differences are small it can become an arduous task to integrate the features. Missing parents, children, references, you name it, it can go wrong. Working on 2 assemblies isn't so bad, and as of I2 multiple users can do different sheets of a drawing and then merge them together later.