Re: TD 8.0 and QC on the same client concurrently??
You can access TD80 and QC8x or QC90 on the same client, but not at the same time : they both use COM objects that have the same prog id and clsid, but are of different versions. In particular, the protocol used between the client and the server seems to have been changed. Merc. did not use the version control mechanism that MS has put in place for COM objects (something like TDAPIOleLib80.TDConnection.1 instead of TDAPIOLELib80.TDConnection). They also sometimes use the class id instead of the prog id to access their COM objects.
Bottom line : You can open a TD80 session (this may load and activate the correct version of the COM objects for TD80), work with TD80 and close that session. You can then open a QC session (this would load and activate the correct version of the COM objects for QC), work with that version and close the session. But if you try to open a QC session while a TD version is opened, it's likely that the loader won't be able to activate the correct version.
Also note that if you have both TD and QC on the same server, you should not open any QC session on that server (as a client), because TD80 uses the client COM objects on the server to perform some operations, such as paste.