Double Neutron Stars ( DNS ) have to survive two supernovae and still remain bound .
This sets strong limits on the nature of the second collapse in these systems .
We consider the masses and orbital parameters of the DNS population and constrain the two distributions of mass ejection and kick velocities directly from observations with no a-priori assumptions regarding evolutionary models and/or the types of the supernovae involved .
We show that there is strong evidence for two distinct types of supernovae in these systems , where the second collapse in the majority of the observed systems involved small mass ejection ( \Delta M \lesssim 0.5 M _ { \odot } ) and a corresponding low-kick velocity ( v _ { k } \lesssim 30 km s ^ { -1 } ) .
This formation scenario is compatible , for example , with an electron capture supernova .
Only a minority of the systems have formed via the standard SN scenario involving larger mass ejection of \sim 2.2 M _ { \odot } and kick velocities of up to 400 km s ^ { -1 } .
Due to the typically small kicks in most DNS ( which are reflected by rather low proper motion ) , we predict that most of these systems reside close to the galactic disc .
In particular , this implies that more NS-NS mergers occur close to the galactic plane .
This may have non-trivial implications to the estimated merger rates of DNS and to the rate of LIGO / VIRGO detections .