I tried this, or at least similar using what I had in. Coarse oats (not rolled) and little mung dahl lentils. I figured the mung dahl would work at least at well as they cook down with a certain creaminess.
For me the lentils were very noticeable at 2/3 oats and 1/3 lentils, so I drizzled the lot with molasses because I wanted a sweet brekky ÷)
However, it was tasty, and I liked the extra protein in there too. I'd do this again but as part of something savoury, and I may still do red lentils at a lower proportion.