I’m pretty sure the answer is no because the number of channels in a layer seems to be set by the number of channels of the first file import, subsequent imports will prompt to convert stereo to mono to edit, so I assume the only way would be to use dual-mono stereo files on that first import and then when you import a true stereo file, it will play as stereo alongside the dual-mono?
You can alternatively open your mono file, and go to Project > Reformat and make it stereo.
But yes, there can only be one format per project (or you would have to work in 2 different project tabs).