We have 2 aircraft

plane A weighs 300 Tonnes and plane B weighs 200 Tonnes.

Both are identical and have the same wing with the same Lift drag ratio of 17 similar to a 747.

Both aircraft are cruising straight and level at 400 Km/h.

Sorry, that is not possible. Identical airplanes with different weights an the same cruising speed can't have the same L/D ratio, because they have different AoA. From then on, any conclusion you come to is arguable.

L=q.S.CL

D=q.S.CD

L/D = CL/CD , that is exclusively dependent on AoA. They can'y fly at the same AoA.

As for the slowing down subject:

The heavy airplane has more Drag than the light one, when speed is constant. When you cut thrust to idle the difference between thrust required and actual thrust (let's say nil for simplification) is greater: greater retarding force than in the light airplane case, which tends to decrease stopping distance. However, the higher weight airplane has more mass, which tends to increase stopping distance. Which effect prevails?

D varies because CD varies, when q and S are constant. CD varies because CL does vary too (they are childrem of the same father, AoA). AoA varies linearly with weight, and so does CL. However, CD is not linear. At lower AoAs, CD varies less than linearly with CL. At higher AoAs the opposite is true.

So, at low AoAs, D will vary less than mass for a given weight change. Inertia prevails at low AoAs or high speeds. Mass increase is greater than the drag force increase and as a result acceleration is reduced.

At high AoAs or low speeds, air viscosity prevails over inertia. Mass increase is less than the drag force increase and as a result acceleration is increased.

At least that is the conclusion I have come to thinking about this. I'm standing by for your comments.