  I explore a triple-star scenario where a tight neutron star (NS) - NS binary
system enters the envelope of a red supergiant (RSG) star and spirals-in
towards its core. The two NSs accrete mass through accretion disks and launch
jets that power a very luminous and long transient event, a common envelope
jets supernova (CEJSN) event. Dynamical friction brings the two NSs to merge
either in the RSG envelope or core. The total energy of the event, radiation
and kinetic, is >10^{52}erg. The light curve stays luminous for months to years
and a signal of gravitational waves might be detected. The ejecta contains
freshly synthesized r-process elements not only from the NS-NS merger as in
kilonova events, but possibly also from the pre-merger jets that the NSs launch
inside the core, as in the r-process CEJSN scenario. This scenario shortens the
time to NS-NS merger compared with that of kilonovae, and might somewhat ease
the problem of the NS-NS r-process scenario to account for r-process
nucleosynthesis in the early Universe. I estimate the ratio of NS-NS merger in
CEJSN events to core collapse supernova (CCSN) events to be <10^{-6}-2x10^{-5}.
However, because they are much more luminous I expect their detection fraction
to that of CCSNe to be much larger than this number. This study calls for
considering this and similar CEJSN scenarios in binary and in triple star
systems when explaining peculiar and puzzling super luminous supernovae.
