I'd say your white snail is one randy fellow. I've had a couple of Mysteries that were constantly after sex; the first one ran everyone ragged and ultimately, I believe, just wore out a female until she died. Off to a breeder he went. The other super-mater met his match in a female and they literally turned the tank upside down with their constant antics. Everyone/thing was upset, from the betta, who was the star of the tank, to the water parameters from all the pooping and egg laying and general chaos. Off they went to the local fish store, with a warning to separate them unless a snail farm was desired.
Now I have only one Mystery per tank. If you go the solo route too, it looks like your white may have sealed his solitary fate. On the other hand, if you give him away and try again with two new ones, you could find yourself in the same situation all over again. Some are super maters, some are average, some have little interest. This goes for them being opposite or same sex, they'll be who they are whether it results in offspring or not.
It's possible that the new blue was young or adjusting to the change of environment, or both, and the white one just wouldn't give it breathing room to adapt. I've seen the "elder" literally take the new one under its wing (meaning antenna, or antler as I call it) and protect and guide the junior until it is an adult. Your white appears to have other things in mind.
Btw, have you checked the top rim of your tank for egg sacs?