There are a couple of ways to do it:
- You can use the ordered transformation here.
- You can add jacobian correction for the volume change in your case above as well to make it proper.
- You can jointly sample from a bivariate standard Gaussian then rotating them by 45 degrees. The rotated sample is transformed into rates that lie in the unit square. See here.
- You can add a potential to restrict the order, see here cell [4] for an example