Does throwing a second smoke grenade make the smoke darker or is the effect not cumulative? Does it depend if it is thrown at the same point or is there an algorithm that calculates which parts of the smoke cloud should be darker?
Disclaimer: I am not a developer of Battlefront and didn't write the engine, but this is what I believe is happening:
Graphically speaking, yes. Two grenades place two particle generators in the engine; the more particles in a space, it considers the area more dense with smoke, therefore more of the pixel will be altered from it's original color toward black.
So it makes it more dark; but since the particles are dense enough to reduce visibility through it anyway, if they are thrown at the same time, you're not going to reduce enemy aim significantly with a second grenade. You'll only affect the color of that already opaque pixel.
Gameplay speaking, overlapping grenades don't make it "worse," depending on the timing and spacing of the two grenades.
Once a grenade is out, it blocks homing completely if the cloud of smoke is between you and your target. Once the timer reaches a certain point it returns to 0% blocking (enabled again). So if there are two grenade effects between you and your target, it can't block homing (homing rockets, droid targeting, etc.) anymore than completely, as it is already with the first. However, if they aren't thrown at exactly the same time, then it will take until the second grenade reaches end of its timer before the homing feature is enabled again. So a second one lets you extend the time of visibility reduction, but not so much the actual visibility.
This assumes both grenades are dropped at the exact same place. If they are offset, and the first dissipates but the second has not, and the second is not between you and your target, then you can "see" through the first (for homing purposes), even though the second hasn't dissipated (since it may be near visually, but technically isn't in your way from a hitscan perspective).
In other words, it doesn't enable homing based on some analog "visibility level" but whether or not the cloud is between you and the target and if the cloud is still "actively blocking." Once the timer ends on the homing blocking, homing is enabled--even though the graphical particles haven't completely dissipated yet.
Needless to say, there is no wind in this virtual sand box, so the behavior is the same indoor as it is outdoor.