In Korg Kronos I have something called “velocity zones” that allows me to fade out a sound around the split point.
For instance I can make a bass / piano split and let the piano go down in the bass area but at a lower volume that fades out at the buttom split point.
If you’re actually reducing velocity rather than just doing a volume cross fade, you could probably do it pretty easily using two midi in blocks, one for each instrument and then set the scaling curves of each of them to reduce or increase respectively as you approach the split point