MU referencing

Hi all, When using MU referencing, how do I code the situation where the typical value appears to be the same between two groups, but one group is more variable - how do I give these two groups different ETAs but essentially the same THETA? Is this legit? tvParam=THETA(.) MU_1=tvParam MU_2=tvParam Param1=MU_1+ETA(1) Param2=MU_2+ETA(2) Param=Param1*G1+Param2*G2 Where G1 and G2 equal 0 or 1 based on which group the subject belongs. Thanks for the advice, Brendan