I have long been a fan of such an idea. This is borne out of experience
of express routes running in the old Met county areas like Tyne and Wear
and West Yorkshire where a multi centred conurbation can support such
services. The other key example which works well is Hong Kong which has
a hierarchical bus service network.

From my experience of it in West Yorkshire, it doesn't work very well.
The Express X84 runs (with a minor deviation) out of Leeds as far as
Lawnswood with the local 1 and 95. It is only express in the sense
that it picks up only outbound and sets down only inbound within the
Green Zone. When I have ridden it, the only place where it passed a 1
or 95 is inbound where the 1 and 95 diverge off the A660 to stay on
Woodhouse Lane and serve the University. Most of the benefit of this
express run was in the driver's sadistic enjoyment of slamming the
doors shut in people's faces in Headingley. I didn't even save any
time as I had to walk down from the Headrow to the Railway Station,
which the 1 would have dropped me near.