I'd expect the pads might get a bit whippy - I've seen some people make braces up between pads.
Depends how tall your stacks are and how you ride. I pull on the tribars quite hard climbing on 100" fixed, and my 60mm 3T stacks are fine. TriRig suggest putting a bridge across (their BTA bottle mount) for spacer stacks over 35mm. Fibrelyte make Pro Missile stacks up to 100mm, but they make a cross brace too, so I'm guessing people using the really tall stacks have felt the need to join the two sides.
I usually use the 3T bridge in front of my Garmin mount, as the first generation Ventus has a tendency to rotate the extensions about the vertical axis, but I've ridden without it and not had any trouble.