Constraining mxAlgebra across submodels

This is actually quite a quick question and the title says most of it already.
Anyway, is there anyway to constrain mxAlgebra objects across multiple submodels?

I would like to constrain estimated heritability of two separate twin models to be the same while allowing each submodel to have separate parameter estimations.

Unfortunately I was unable to find a way to do so. I have tried mxConstraint(modelA.algebra1==modelA.algebra2) but that seemed not to work.

I would be thankful for any reply!