This was the idea behind the late-Cold War "high/low" concept that gave us the F-15/F-16 in the US Air Force, and F-14/F-18 in the US Navy. Initially the F-35 was supposed to be the "low" companion to the "high" F-22, but, with the end of the Cold War, that changed.

One major issue that the US has run into with deploying that strategy, in the absence of great power opponents, is that the largest long-term costs are not the airframes, but pilots and pilot training. An F-16 pilot isn't cheaper than an F-15 pilot, and getting hours for an F-16 pilot isn't cheaper than getting hours for an F-15 pilot, other than the lower fuel cost for the F-16.

That said, the F-15X that the USAF is procuring is basically going to be used as a lower-cost-than-F-35 "missile truck," so not incompatible with what you suggest.