Biasing only one particle

Good evening,

I would like to ask if we could perform biasing of only a single particle per event.

I would like to enforce a specific type of interaction to occur for an incoming muon across a target volume. So, I have a mu+ beam, a carbon target, and I am trying to perform biasing with the MuPairProduction process. But how could I simulate this biased interaction in the presence of pileup? Imagine that there is a second muon entering during the same event (there is an external generator). Is there a way to tell Geant4 to bias only one of the two muons?

Thank you very much in advance!