The ability to have different investment directions for non-Roth and Roth and/or different sources is baked into most recordkeeping systems. After all, once it's coded, it shouldn't be a problem.
The challenge and difficult conversations come up when the participant decides to invest the assets differently and then wants to split the assets up between non-Roth and Roth to move forward. Of course, they always want the best performer to be in the Roth side of the equation.
So to answer your questions above.... maybe typical, maybe not.
I've worked at a company that opened the sources to different investing. I'm currently at a place that does the one investment strategy for all sources.