[mixture models] assign individual to a subpopulation every dosing record?

Dear NMusers, I am currently trying to use mixture models on my data because the absorption rate constant (Ka) appears to follow a bi-modal distribution. But what makes this complicated is that each subject has two dosing occasions with a washout period in between, and the absorption tends to vary a lot for the same individual (the extent of inter occasion variability of Ka is similar between individual variability). Therefore I would like to allow Ka to vary for the same subject during different occasions while keeping all the other parameters the same for a certain subject. I know I could use inter-occasion eta term on Ka, but I want to allow Ka to have the ability to be assigned to different subpopulations for the same individual. But in the mixture model structure, the $MIX is called once per individual and we can only assign each individual to a subpopulation once.. I wonder if there is a way to assign individual to a subpopulation every dosing record? Thank you in advance. I appreciate any comments or suggestions. Yu Graduate student in Clinical Pharmaceutical Science University of Iowa