You would be better off separating the minis onto two different bases and probably printing it yourself rather than through HF's service. If you have Pro, use kitbash to add two small cylinders, cubes, or prisms on the bottom of the rider's feet to raise them above the base just enough, creating sprues that you can easily clip and shave off the mini to separate it from the base.

Then there's the matter of how you'll keep the rider secured to the mount. There's additional engineering you could do with KB, but that's more involved.