To link parameters of the models to be treated as one variable we use link_parameters() method. Relationships are transitive, so if parameter A in one model is related to B in the other (A=B) and B is set to be related to C elsewhere, the transitive relationship A=C will be automatically established later. All we need is establish an initial 'chain' of relationships between such parameters. It is possible to have parameters of one model linked together (say, in complex models some adjustable parameters in different parts of the equation may be treated as one variable).
For specific usage see TotalFit.m