I had a combo many years ago (that is why I have an opinion on this) that was exactly what you are talking about. You turned the handle around. Had a hole at both ends then a cap to put on when you picked one or the other. As you mentioned, was a great spinning rod, but lack so terribly as a fly rod....40 feet? Not bad but I am betting that is real work.
At least ORVIS offers two different butt sections in their combo, the top to sections are the same for both, but they saw the need to address that stripping guide issue.

I am not saying this all won't work, it will, but it will not be at top form for the fly fishing.

But, I haven't needed to pick up a spinning outfit for over a quarter of a century, so I am bias.