I don't think you can access to different versions of QC from one client simultaneously, even with using IE and QC Explorer.
With v10 the DLL that is the QC Client is downloaded through IE each time you access QC. I assume it is also downloaded when you use QC Explorer (but I don't use that tool).
Each time you access QC (at least through IE) it will check the version of the downloaded client against the version of the server. The version changes with each patch. If the versions don't match exactly, the client will be re-downloaded.
HP has documented that not until ALM v11.0 can you access two different versions of ALM QC simultaneously from one client.
With v10 the DLL that is the QC Client is downloaded through IE each time you access QC. I assume it is also downloaded when you use QC Explorer (but I don't use that tool).
Each time you access QC (at least through IE) it will check the version of the downloaded client against the version of the server. The version changes with each patch. If the versions don't match exactly, the client will be re-downloaded.
HP has documented that not until ALM v11.0 can you access two different versions of ALM QC simultaneously from one client.