Specify constraint on sum of parameters

Is there a way to specify a constraint on the sum of two parameters when doing bayesian estimation?
For instance, I have two parameters and in estimated_params block, they’re declared as

a_y1, 1.4, 1,2 , uniform_pdf, 1.5, 0.083;
a_y2, -0.597, -1,0 , uniform_pdf, -0.5, 0.083;

what is the syntax to specify the constraint that 0<a_y1+a_y2<1? Thanks!

The general answer is complicated, but my guess is the specific case can be solved via Estimation AR(2)-process/Joint prior distribution

very useful information. thank you!