[R] question about gamm models

[Matthias an der Heiden]

Dear R-Users,

I have a question concerning mixed models in R.
The strata of my model are the counties of Germany. The differencies 
between these counties should be modelled as realizations of a normally 
distributed random variable X.
Moreover, the model contains a 0/1 variable A that enters as a fixed 
effect.
The only special feature that should be additionally in the model 
is the following:  

In a usual mixed model the (constant) variance of X will be chosen 
in an optimal way, but I want to fit 2 constant variances, one for 
the subset {A=0} and the other one for the subset {A=1}. Nevertheless 
it should be one and the same random variable X.

I know that it is possible to fit a model with two independent random 
variables X1 and X2 for the subsets {A=0} and {A=1} respectively.
But I want it to be the same! Equivalently the correlation between 
X1 and X2 should be 1.

Can anybody help me in this respect?

Yours sincerely     Matthias an der Heiden