Particle A is spinning one way in a cyclotron,B the opposite way both at 99.9% of C .When they collide is the energy released at their relative velocity of C or that of double C?

Velocities don't add linearly

If they are the same particle type, i.e have the same mass, then in your case the collision will be stationary in the lab frame (ie. to a scientist standing watching). Notice that colliders are not always like that: the LHC for example will be colliding gluons whose cntre-of-mass frame will not be stationary (because the gluons have differnet energies).

The relative velocity, V, of the two particles (each with velocity v) will be given by the formula V=2v/(1+v^2/c^2), which is still less than c. The energy involved in their collision depends on their energy, which is not directly related to their relative velocity V, but is given by E=2mc^2/sqrt(1-v^2/c^2).