I have to perform a linear regression of the form
y=a*x1+b*x2+c
where
b=(1-a)
where $y=ax_1+bx_2+c$ where $b=(1-a)$ and I have errors both on the dependent variable y and on the predictors x1,x2$x_1, x_2$. I am not sure how to handle the constraint on the second coefficient and the errors on the dependent variable. Any idea?