As REH was definitely one of the prime co-creators of the Cthulhu Mythos (as we now call it) he and HPL seemed to freely give and take from each other's concepts and, rarely, styles. I have no doubt that REH set his stories in what he considered the ancient history of HPL's version of Earth.
Whether that was a good thing or a bad thing for Howard himself has been part of the debate(s) about REH's stability since his suicide. Of course, there have been many, many rumors about Lovecraft's sanity too.... (heh heh heh!)