Well, trotting up to the sixth floor wasn't in the Three Amigos plans. I think that can be said with a high degree of confidence based on their testimony.
Here's a diagram of the fifth floor
Photograph of NW corner of fifth floor as viewed from where the Three Amigos were looking out the SW corner window
I can speculate on what the trotter trio might have been thinking but that's about as conclusive as I'm willing to go. The limo was gone by the time the trotter trio looked out the SW window and they could have assumed that the sniper(s) had left the sixth floor by then so they decided to split the scene. Let me remind you of your own admonition. Don't assume what a given witness would do at any particular time
I don't think I'm assuming anything about what they should have done. All I ask is were they in a position at the west windows to be spotted by an assassin in flight, who for all they knew, had reason to pop onto the fifth floor.
I can see why they wouldn't ascend to the 6th floor. That would be foolhardy. But did any one of them testify that they figured the killer had gone because the limo had left the scene? You seem to be suggesting that. And by 'splitting the scene', if you mean trotting over the the west windows on the floor right below the floor that would require the killer to pass by, I'd suggest that they were indeed still smack-dab in the middle of a
clear-and-present danger, arguably a potential life-and-death situation.