I do like radical thinking, whether it turns out to be practical and workable or not, it is what takes all kinds of developments in completely new directions.
I'd agree, feeding a turbocharger into a supercharger is indeed compound charging providing there is a pressure ratio higher than 1 across both at any time. There is no difference in doing this than doing the same thing by feeding one turbo into another.
The twin engine arrangement is also something that has been done many times on everything from rallycross Minis to Pikes Peak twin Ottinger-engined VW Sciroccos.
In the UK it isn't legal to have both engines running at the same time on a road car though.
This can actually be very straightforward on a FWD car, and using second hand parts/ donor vehicle not at all expensive. It would likely cost more to get the extra power out of the second engine than to do the whole rest of the conversion if it is done skilled DIY style.
Reliable.... of course. 2 gearboxes, 2 differentials and 4 driveshafts acting on 4 tyres, a fair bit more total car weight, but also twice as much power to push it along.... or looking at it another way, each engine only has just over half of the cars weight to shift.
As already mentioned, a manual gearbox in the front and an auto behind seems to work well to allow for speed differential, but 2 manuals can also work if the linking is done well.
I've even seen one case where the driver could apply more throttle to the rear engine to help kill understeer in corners by using a seperate additional rear hand throttle!
It should certainly accelerate quickly, but the down side is the extra weight generating higher lateral loading on the tyres in corners..... to the point that it could actually be quicker overall to build an ultra-light car with only one engine?
It would certainly be interesting and fun to drive though!