Now that an interesting Idea Terry and quite similar to something I was playing with.
But from a different angle. I like the idea of the two lengths though and it make for a potentially very versatile rod that would switch from the river to the loch easily too.
I've built a few Euronymph style rods now and the one that seems to be coming out as the favourite is the North Fork Composites 11' 2wt. Now as a euro nymph this is very much a 2 and happy with a 3, but its very light In the hand and has a very fast tip that bends progressively with a fish (of any size)
What I've noticed is that if you overline it then it seems to be a really nice 3/4wt roll casting/spider/light switch and potential drifting a team in a boat rod.
I've only cast it like this on grass so far but I think it has real potential as a light boat rod, just need to get another one built up to try out in this way, this year.
Would be interesting to see how the two compare when the time comes. as it might suit different folk.
Your idea works for someone fishing the river with dries and spiders who then wants a longer rod for a boat. (more what I would do)
The NFC is euro nymph rod that can be overland to become a very nice light long boat rod (which might appeal to some of the Comp guys)
Looking forward to hearing how you get on