@ mrsrich
You can not mix and match at all.
QBO- you can be invited to sign in as the accountant, then you can log in to your clients books when ever you want.
QBDT - You either use a remote log in software, which requires that the client computer be on and no one else using it (I do not know how that works in a multi user environment), or they send you an accountants copy (assuming you have the accountants version of QBDT) you work in it, save it and send it back - they then import the copy you return and your entries are entered.
You can not accurately create a QBO file from desktop, load it in QBO (assuming you have another subscription to do so) work on it, export to desktop and they import the changed file - just does not work.
There is a different skill set too between QBO and QBDT, basic fuctions are the same but names are often changed to protect the innocent as they say. And desktop is much more functional than QBO.
QBDT - you need the same year QB as the client has, unless you have the accountant version, in which case the accountant version can work in a file that is one year older and still save the accountant changes in that one year older format.