Combining wires from two breakers to one appliance?
As far as 40 vs. 50 - that depends on the manufacturer's requirements. The installation instructions clearly state:


  A 50 Amp circuit breaker with wire gauge #8 AWG must be used.


So there you have it. 50 Amp - not 40 Amp. 8 AWG - not smaller. You can, of course, use larger wire - e.g., 6 AWG - that is always OK. But you can't use a smaller breaker - my guess is you would get nuisance trips if you use the microwave + convection in the top at the same time as you use broil in the bottom oven. And you can't use larger because it would not provide the necessary protection in certain situations.

As to why the oven needs a 50 Amp breaker if it seems to only use 7.8 kW, only the manufacturer can say for sure. But a couple of possibilities:


7,800 W/240 V = 32.5 A. 32.5 x 1.25 = 40.625. That is just over the magic 40.0, so goes to the next breaker size. But that would seem like a very poor design to be "so close".
7,800 W/208 V = 37.5 A. Of course, if the reason to need a 50 Amp breaker is for 208 V installation, they could have listed 40 A for 240, 50 A for 208. But they didn't.
7,800 W is the "usual" continuous rating, fitting OK (or close) with 40 Amp, but maybe the surge (startup of top &amp; bottom ovens at the same time, part of self cleaning cycle, etc.) is significantly > 40 A, so 50 A is needed to avoid problems.


In any case, you must NOT combine two wires to get more capacity. It is against code and, aside from any other technical reasons, the basic problem is if one wire broke (or more likely, one end came off a connector) then 50A would be flowing over a wire that can only handle 20A or 30A. You can't parallel, as logical as it might seem.