We use only the connection string from DataConfig.config
and we do not look into connectionString settings in the root web.config file. We have not performed tests about scenario where you could have two modes of the DataConfig file., but I agree that it would be useful. You could try using post build events of the project that can be set through Visual Studio. There you can define a file ( DataConfig.config) which you can replace depending on the build mode.
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