I am working on a problem in the PDE Equation Modeler. In my geometry, I have an identity pair for which I would like to impose a custom boundary condition. Is there a way to do this in Comsol. I have defined one side as src and one as dst. The BCs I would like to use are below:
On the src boundary: 
On the dst boundary: 
where A, B, C, D, E, F are constants.
Is there a way to set this up as a boundary condition? I think I could use a flux/source boundary to set up the left side if the right side were equal to zero, but I haven't been able to figure out how to reference the u values on the other boundaries.
Thank you for any help.