Experience shows that they are very similar, yet as concerns
BEING united, we stand; BEING divided, we fall.
united, we stand; divided, we fall.
united we stand = we stand united (no comma)
being united, we stand is a bit different.
It could mean that while/when someone is uniting us, we stand, and otherwise we sit.
Let's see what native speakers have to say on this.
Student or Learner