I'd like to sample a pair of continuous parameters which has the constraint that one has to be smaller than the other one. I understand one approach is by rejection sampling by rejecting the samples that are violating this constraint. Another way would be propose a pair of parameters using a 2D Gaussian distribution that are centered at the current sample, and assign the bigger value to the first parameter and the smaller one to the second. However, I'm not sure if this still keeps detailed-balance property. Should I compute the forward and the backward probabilities? Any ideas would be appreciated.