Is it possible to have multiple Teradata Drivers installed on a single computer and specify different connections to use different drivers?
Which Windows System environment variable determines which driver .dll is used for a Teradata .NET connection if I don't want to use ODBC?
Maybe I am miss understanding the question. But you can have Teradata .Net and the Teradata ODBC driver installed concurrently. The use of each is determined by the connection method employed by the application or by selecting the appropriate method in SQL Assistant.
If you would like to have multiple versions installed of the ODBC driver I believe you are going to find that a bit more tedious. From what I can recall the installation instructions for newer versions of the driver explicitly state that all remnants of the previous version should be uninstalled to avoid conflicts or problems. This may be due in part to the GSS and ICU libraries that are required.
From a pure testing point of view this would be a case where having several virtual machines at your disposal using VMware or VirtualBox would be helpful. Each VM could be configured with a different version of the Teradata Client you would like to test. (e.g. Teradata 12, Teradata 13, Teradata 13.10) I think it would be excessive, to a degree, to have different VMs for minor revisions within a release.