how do we combine velocities of multiple volume grids in 1 grid just like we do densities with vdb combine.

right now I have 11 grids, densities are combined with vdb combine nicely, but with velocities doesnt seems to work that way..

I think I figured out a way by splitting all velocity elements into x, y and z respectively , then combining all x, all y, all z and merging it them back with the density worked for me.