keeping face index

Hi friends,

I have a lens that allows for two people, with different hats.

Ann comes into frame first, and is assigned face index 0, with the hat that goes with it.

Bob comes next, and gets face index 1

Then Ann looks away, is lost to the software, and Bob immediately gets face index 0.

Is there any way he can keep his original index 1?

And so when Ann reappears, she gets her old index 0 back?

Thanks,

Alan

(If they both lose their face, then it's their own silly fault and they can sort it out themselves!)