It's quite possible to have the apparent rate of separation of two particles, from the standpoint of third frame of reference, be superluminal. It happens in astronomical observations. See http://spiff.rit.edu/classes/phys200/lectures/superlum/super... for a nice example and explanation.

But you are quite right that from the frame of reference of either of the two particles sent traveling at .75c in opposite directions, the other particle is receding at subluminal velocity, not FTL.