Not trying to be difficult, just don't really see the difference between layering them and mixing them. They're still touching each other. Plus, the foundation has SPF in it, too, so dilution wouldn't seem to be an issue.
Do you have an article or source I could look into a bit more?
Google seems mixed, but most sources seem like personal experiences.
From Paula's Choice FB: Can you mix your tinted moisturizer or foundation with your sunscreen, i.e. to counteract the white cast of a mineral formula? Yes, as long as your formula contains an SPF 15+, you can mix in either your foundation or tinted moisturizer with your sunscreen. If your foundation/tinted moisturizer does not contain an SPF 15+, you can mix a few drops of it into your sunscreen, but no more than that or you risk diluting your sun protection.