Can I link models or studies via an internal variable?

Hi,

I'm struggling with a diffusion-only problem using dilute species transport. I would like to set the flux of species B into my system by setting it as the equal to the negative of the final (stationary solution) flux out of the system of species A across the same boundary. The two species do not interact in anyway and I am interested in a stationary solution.

I have tried using (i) a single model with two species in my physics node, (ii) two physics nodes (one for each species) in the same geometry with two (sequential) solvers and also (iii) two models, each with the same geometry but with two studies.

I cannot find a way to set the normal flux out of species A as the inward flux of B as:
in case (i) ntflux_cA is evolving at the same time as ntflux_cB
in case (ii) ntflux_cA doesn't match ntflux_cB but I cannot see why
in case (iii) study1 (for species A) doesn't seem to make ntflux_cA available to study2

Any thoughts would be appreciated.
Stuart