Multiple gamma source with a defined angular distribution

Hi how can I generate a coincident gamma source with the following scheme

  1. Emits 2 gammas of energy e1 and e2 in coincidence
  2. gamma 1 is emitted isotropically
  3. gamma 2 is emitted from sampling a user-defined distribution for angles(measured with respect to gamma1)

Any help and suggestion in this would be very helpful .