Seprate NVT equlibriation of two type particles in two regions

Dear All,
I want to canonically equilibrate two type particles which have different mass separately in two different regions.
For this i have tried fix wall/reflect but they are mixed at the interface. How i prevent this mixing.
Thanks in advance.

Regards

Dear All,
I want to canonically equilibrate two type particles which have different mass separately in two different regions.
For this i have tried fix wall/reflect but they are mixed at the interface. How i prevent this mixing.

Simply run an MD on one group of atoms only, unfix the integrator, and then apply it to the other part and run some more.