I've run that 'nested' course configuration for club regattas where we have faster and slower fleets.  It works well as long as you have good mark setters.

The request to have the finish between the nearer and farther windward marks is indeed challenging and it would be simpler (and safer) to have it in the lower 1/3 of the course.

An alternative you might consider would be to have everyone sail the same course, but give the F18s three laps while the H16s sail two.

Anthony ... why not a classic diamond with the SL/FL in the middle?  You can still have 2 windward marks to separate the fleets. 

Why does the finish need to be restricted? What is the concern that you are trying to address?

You could lengthen the course at the bottom.  Two leeward marks or gates, appropriately spaced, with common start line and finish line.

I reckon I can learn a lot from some discussion here.

Why do cats want an extra windward leg?

I'm guessing, but what I think they are trying to achieve is to avoid having the H16s sail a longer windward finishing leg up to the F18 windward mark, while still giving the F18 a beat to the finish for whatever masochistic multihull reason.

A real problem is that F18s and Hobie 16s sail different angles, both upwind and downwind. F18s have a huge blind spot behind the gennaker.  Closing speeds are high in any significant breeze.

I have never seen a finish line between the two weather marks.

It would be much safer to have the upwind finish above the long weather mark.  The only real reason to have an upwind finish is to put the fleet closer to home on the last race of the day - both fleets would need to sail that direction anyway.  Both these fleets sail upwind just so they can sail downwind again - they generally don't want an extra upwind leg just because.

[Just as an aside, I raced Hobie 16s from 1973 until 2016; F18s from 2004-2012 and have been the Race Officer at both their North American / Canadian / US Championships more times than I can remember. I've raced on this type of course many times - without the finish line between the upwind marks.]

I have run hundreds of races for mixed fleets involving F18s. Downwind finishes for F18s is a bad idea. Very unsafe... you'll have F18s with their kites up (and the associated blind spot) going full speed into an area full of other boats milling about waiting for their next start. Longer windward marks for the F18s (usually at a range of .7-1+ NM) on two-lap races is a staple of our practice. We usually run separate start and finish lines off different sides of the RC boat in the bottom third of the course. Nobody seems to complain about course length or enough windward sailing.

I am with Matt and Dominique on this. the Hobie fleet has a course sticker which give the RC opportunity to change up the courses during the day using both upwind and downwind finishes. Sailors do enjoy the upwind finishes. Upwind marks at least 2/3 of your upwind distance, leeward mark at least 1/3 below RC vessel. Separate WW for F18 and if it's a large fleet, use an offset for them. (safety reasons). If you're worried about boats sailing through the finish line when not finishing, many of us have language for the SI's that cover that off that can be shared. The main reason for making the FL an obstruction is as Dom suggested, you have boats either waiting to start or in a starting sequence in that area and you don't want a fleet charging through there at the same time. Safety! You don't need to dsq, you can give them an out with a turn. 
Another option which I use when feasible, especially if it multiple fleets is a separate finish line away from the starting area. If it's just the two fleets, you're ok with S/F off the RC vessel.