You can define your logp manually in a CustomDist if you want to use Censored Normals but treat the x,y pairs as non-exchangeable.
However I am not sure if Censoring will make complete sense in this case, since it will be axis-aligned? What if censoring happens when x+y>b?
And are you sure you are working with censoring and not truncation? Sounds interesting either way.
Do you have an example of code that generates synthetic data?