Great concept, I would not have a T tail, I can see from the drawings
that you are trying to let the rudder and elevator run in as clean air
as possible.that has obvious advantages, but a couple of disadvantages,
one of them is structural integrity, and just by eyeballing the big
elevator and rudder surfaces in this design, and then looking at the
size of the body between the two propellers....thats an awfully thin
stucture, that will hold that T tail.

One of the disadvantages with letting the rudder surfaces run in clean
air is less rudder authority, thus the need for increasing the surface areas.

Having that little structure on the boly at the location of the propeller
arc....., in this case I would suggest to go the other way. make an
ordinary elevator and rudder, and let it ride in the propeller wash.

You will get higher drag, as the airspeed will be higher in the propwash
, but instead you can go down in size of the surfaces, so the drag will
end up being the same.

Second advantage of this is rudder authority,  You can get it down to
standstill, and that is something that can come in really handy come
crosswinds.

Some designs did a point of getting as much rudder authority as
possible, and especially propeller twins or 4 engine aircrafts, had
their rudders at the end of the elevator, , so they looked like an H ,
that is actually not a bad idea.

One engine stops, and you are coming in on landing with one engine
running, are getting on the slow side, and have strong crosswinds, those
big huge surfces  sitting in the air ( that you need to get rudder
autority in free air), are now losing authority more and more, but are
instead catching more and more of the crosswinds.

If you are trying to minimize adverse jaw at a one engine loss, look at
ME110, look at B-24.... Big rudder autority on small surfaces, right in
the midst of the propellerblast.

With those long slender wings, and slick design with a good
powerpackage, you can do some serious altitude with this thing, however
your propeller diameter, and high altitude capability are in conflict,
with that wing arrangement and with that power, you can probably go as
high as when you see the sky blacken, and see star constellations in
daytime.

That will make for some serious speed and distances.

There is a lot to propeller science, but as a very broad generality, big
slow blades are more effective than small speedy things.

This may... and I say may be a concept to pursue.

Of all engines, none fit the bill better than a rotatry engine to do
this. ( that I know of , up to date) . To connect two engines, through
the crankshaft is considered very hard.  First  because of a balance
aspect, most other type of the few engines that can do it,  because of
that, needs to line up in a certain way, or be an industrial heavy type
with a lot of cylinders to make this possible,second, there are not many
type engines that because of a long line of technical reasons are suited
to be connected in series. In some very big earhmoving equipment and big
huge industrial Catepillar applications this is done.

Having not done it, but seing what I see, I would say that the prospect
of using two rotatys, and connect them, where one will run through the
crankshaft  (or center shaft) is very high. Because of it's perfect
balance, there would not be any need for synchronisation gears, to make
it connect at a certain position....most probably the only thing needed
would be a simple clutch.

The power transfer in a rotary does not go through a crankshaft that
looks like a snake, it's a straight shot.

You would use the rear engine (of you use a tractor type configuration)
in takeoff and climb and fast cruise, stop and  disconnect the rear
engine at economy long distance cruise. In case of an engine failure,
you would use the other engine. If the rear fails, you just disengage
the clutch, and run on the front. If the front fails, you keep the
clutch engage, and run both engines on the rear engine, you will run the
prop via a dead engine in that case, so lubrication to the front engine
needs to be maintained.

Connect and disconnect procedures would need to be worked out, but as a
dry clutch is pretty forgiving in speed differential, just two dials
with RPM for each engine, match the RPM, and engage..will probably be
enough.

If that could be done, you could get rid of the two motor gondolas on
the wing, getting one straight push/pull vector and don't worry about
any adverse yaw at engine loss. you will get power from the other engine
still, and make a tractor or pusher, two engines, one big huge
propeller, one gearbox.

Because of the huge power diffeneces you will get, you can not go with a
fixed prop in this application, the prop has to cover a much broader
powerband

You start and climb on two engines, then get a looooong range fast high
flying aircraft sipping fuel on one engine.

You most probably will get an intercontinental aircraft in this way,
perhaps even the Pacific. You may even be able to take on so much fuel
that you have to wait for a while to be able to maintain altitude on one
engine.....oh man..... you will get long legs.

If the engines can be done this way, you will save a lot of aerodynamic
worries.

The worries you will get with retracting long gearlegs...in return for
what you get....I'll take it.

Not all aircraft styles will lend itself to this engine concept, your
style, almost a glider type aircraft,  certainly fit the bill perfectly.

I am pretty sure that there are very few things that have not been
done, so I am sure someone somewhere have tried to connect two rotaries
in tandem, but I don't know if it has been done for aircraft purposes.
IF and that is IF it can be done, it probably require the least possible
arrangement of all the existing engines, in order for it to be accomplished.

Structurally I give it a very high chance. I don't know if it can do it,
but I really don't see anything that says that it is impossible. On the
contrary, I see very good chance of success on that venue.

The answer that you need to find out is...can a rotarys center shaft
accept twice the load reliably?

Roger