Hi Gregor,
You are correct, when both PRT Loaders have the same time offset and PRT sequence, their particles should be completely in-sync and produce a dot product of 1.0 everywhere, since we are using the Index channel of the one to query the other.
The approach will NOT work if there are particles being born and dying, because the indices would go out of sync when you set the offset of 1.
You could work around this by using a NearestParticle operator which would generally find the matching particle in the offset sequence. If the particle it finds is farther than some tiny threshold (based on how fast the particles move per frame), you could assume the Dot Product value as 1.0 instead of calculating it.