In reply to SB.

I haven't done it myself, but I think you can use PROC NLMIXED for this analysis. For the fixed effects, you can model Y = lambda*X1 + (1-lambda)*X2, where X1 ~ N(mu1, sigma1) and X2 ~ N(mu2, sigma2). That would be 5 parameters for the fixed effects. In situations like this, convergence might require good initial guesses.