>>Why do you want them to use the same instance? Even if it could be done (I don't know how it could), I don't think its a good idea- one application doesn't know what state the other application left it in.
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>I'll be using the COM to hit SQL Server and I could have dozens of users accessing at the same time. My concerns are 1) What is the impact on the server and performance to having so many COM instances opened at the same time, and 2) It would get expensive to purchase dozens of SQL Server client licences.

I think that you will just want to have a MTDLL server. As far as licenses, if your data is being published on the internet by a COM object, SQL server has a special license for this. I would check the SQL server site for details, or call MS to be sure what licensses you need to buy. Either way, your server config won't change your legal obligation to MS to purchase the correct type of license.