If there are only 2 x 750mm exit widths then the occupancy should be 100 persons.
To run at an occupancy of 500 approx would lead to an evacuation time of approximately 12.5 minutes.
BS9999 could potentially be used but no exits below 800mm are allowed in BS9999, and even if the exit widths are bumped up to 800mm, and you have an alarm system providing a clear benefit, and the ceiling height is 10m or over, you still only have an occupancy of 242 under BS9999 according to the exit width available. Also, BS9999 says nothing about controlling numbers through management, it leads you down the path of designing the doors to the potential occupancy based on floorspace, which would (given the maximum deviations allowed) require approx 6.6m of exit width after the largest fire exit is discounted.
Also, if BS9999 is used then it should be used in its entirety, they shouldn't just pick out the bits that suit them.
I personally think that BS9999 is prodominantly pants (sorry for using technical terms) and has many potential pitfalls in various areas. However, many FRS and BCO bodies will accept BS9999 based solutions at face value. (ie. If you have followed it to the letter then it must be right, or at least if something goes wrong they can blame BSI)
The only other option is a full ASET/RSET comparison, ie Prove through fire engineering that people can evacuate the premises before they are at risk from the effects of fire.