Is it ok to only use the top latch on a verticle panic device if it is in a fire wall? Or is it required to have both top and bottm latches positively latching?

I think I understand what you are saying, the bars that are operated by the push bar, engages with the top and bottom of the frame. How well they engage is all about security as far as fire safety is concerned it is how easily they open, if they didn't engage at all, this wouldn't affect the means of escape, but the occupiers would not be very pleased for security reasons.