Only rear cross lever on bullhorns
Personally, I would install both levers for safety.

No matter how careful you are, eventually you will get in a situation where you need to perform an emergency stop when your hands are on the tops, even if you are travelling relatively slowly. (I have experienced inattentive pedestrians stepping directly into my path only a few feet in front of me.)

In an emergency stop situation you will want to brake with both wheels, and the front wheel provides more braking power than the rear.