I agree with Shaun, you do not need a new PW datasource in this case, just setup a separate Managed Workspace / Workspace Profile to use different units.
However, you have to be careful, and make sure that any schema structure changes, you made in your original Metric schema are also included into your Mixed Metric ones, because you will be using the same database structure (created from your Metric schemas) to store components within OPMS database. That is, if you have created any new Classes or added/removed properties to any of the already available classes, you need to do the same for the Mixed Metric project. You can have different tagging format or other things, which do not change the structure of the database though.
Justinas