Okay so maybe devils advocate here, but if we’re okay that you can get the same aerobic conditioning from essentially splitting the long ride in two, let’s look at @buzz point on body position.
I’m not really sure how you can measure this, but holding that position for 2-3h has to stimulate adaptation to some degree, is doing that twice going to be significantly less than holding it for 4-5h? In fact isn’t there some injury risk unless you’re already capable of it?
If there is a gap, can strength work cover it?
@wheezy has a good point about recovery too, like the long run, the long rides can burn you out.
I see one kind of obvious downside, if you’re training six days a week and split your long ride, you’re likely training seven days a week . But maybe there’s ways to change things up with two medium rides.