The brightness of the other planet would be exactly the same as what an observer would see while standing on it looking down at the ground. Or, if you are worried about the atmosphere scattering light, exactly the same as someone flying a plane high in the atmosphere near space, looking down at the ground.
Why? Because the brightness of extended objects is unaffected by how far away from them you are. Extended objects are objects bigger than a single point -- your hand, the ground, the moon, the sun. Basically everything other than far-away stars.
There are 2 things which cancel out:
- As you get farther away from an object, the amount of light from a single point on that object is diminished by 1/r^2.
- As you get farther away from an object, the amount of "stuff" that one "pixel" of your eye sees (e.g., the amount of area seen by one photoreceptor) is a cone projected out into space, so the area at distance r grows like r^2.
These exactly cancel out -- as you get farther away the individual points send out less light but you see more points for each eye photoreceptor, in the exact same proportion, so there is no change.
You can of course observe this in your daily life -- your wall does not get brighter or dimmer as you get closer or farther. Interestingly, this means that the brightness of the sun doesn't increase either as you get closer -- standing in space right next to the sun it is exactly as bright as seen from Earth (or, from just outside the atmosphere of Earth anyways). Of course it will fill the entire field of view so the total energy received by your eye will be much higher, but the per-pixel perceived brightness is the same.
One final notes -- at some point your other planet will be small enough that it is no longer an extended object, and becomes a point-like source like a star. But this will only ever decrease the brightness, since you are now getting the 1/r^2 falloff without the increased area of the object per-pixel.