I have read several scripts of late where they have it written as two dialogues, side by side.  I assume this is proper format because it was in professional scripts. What does this mean or indicate when dialogue is written in this fashion.   Marvin.

It means the two characters are talking at the same time.

Tom, well that makes sense, thanks.  Marvin.

I do it in The Key. They also say the same thing. A more skilled writer who calls himself Charlie Kaufman also does it in Synecdoche.

As Tommy P says, from his vagina shaped cave, it's when two characters speak simultaneously. I use FD, it's quite accommodating  for this type of thing.