Is it possible that when 2 users are updating the same page at the same time, same HTML module, the rad does not allow uploading files?

It happened that I was working with a group of users and some of them were trying to update a page's information, by editing the HTML module. One of them couldn't see the "Upload" option enable when selecting the "image uploader" of the RAD. Could this be because there were more than 1 user working on that page or does it have to do with maximun size upload, maximun persons updating the module, etc? If so, where should I update those conditions?

Thanks!