2D-umbrella sampling and 2D-WHAM

Hi, I am running umbrella sampling simulations using two distance based reaction coordinates in gromacs. I have calculated the 2d-free energy surface using Alan Grossfield’s group WHAM (weighted histogram analysis method) code. However, I am confused about the choice of force constants for each reaction coordinates. For 1D-umbrella sampling, we get a series of histograms for each umbrella window and by plotting them we can observe if the overlap between adjacent histograms are good. However, for a 2d-umbrella sampling how to assess the choice of force constants, as in this case we will probably have a 2d-histogram kind of thing. It will be helpful if you can help me with some workaround for 2D-umbrella sampling histogram analysis and choice of force constants. Thanks