Currently the mstestrunner plugin only supports the /testcontainer: commandline argument for mstest. This quickly falls short when you want to have a project with multiple DLLs and don't want to tweak the build config everytime a new test dll is introduced.
With Visual Studio and mstest and .vsmdi file can be created. This is an XML file that Visual Studio generates and lets you build test suites. Once you have this file created you can use the /testmetadata: commandline arg to point to the .vsmdi file and not specify the test dlls.
See http://msdn.microsoft.com/en-us/library/ms182489.aspx for full details on MSTest commandline args.