when two neutron stars collide, do they creat both a balck hole and release gamma rays or does it happen in stages?
The great majority of neutron stars detected to date are less than 1.5 solar masses. If the actual TOV limit is on the high side [~3], colliding neutron stars would generally be incapable of forming a black hole. The least massive black hole detected thus far is around 3.8 solar masses.
A curiousity is the large discrepancy the least massive black hole and most massive neutron star - about 2 solar masses. As twofish noted, the equation of state may favor lower masses [~ 2 solar] for black holes, and, this discrepancy is due to difficulty in detecting black holes near the lower mass limit.