First of all you mean the DF Client not dfGUI. Technically no, it is possible to start two at the same time if you time it correctly but if you start one and wait a few minutes then try to start another you will get an error message from the second saying that the client is already started (it detects foldtrajlite.lock in the directory).
Since both clients would be trying to write filelist.txt and buffer files, my guess is that things would get messed up eventually.
Jeff.