how to have 2 textures on a sim

I have a friend that owns half a sim and wants the ground texture on her half to be different from the other side, is this possible and how do i go about doing something like this.

I could say this is not possible however I will say I don't think this is possible.
Only a whole region can be changed.

still should be possible, even to be able to "paint" textures on that would be dope lol

What you ask for is 100% impossible. Here is why:

Sim textures are defined sim-wide by elevation, in 4 bands from lowest to highest. The borders between these bands are defined as the height of the transition point from lowest to next lowest, and the transition from next highest to highest, at each corner of the sim. The transition between next lowest and next highest is the mid point between the two defined points. And for all of these, the transition is blended semi-randomly by the server. Between these defined corner points, each transition layer is like a rubber sheet, stretched between the 4 corners. Where the terrain itself passes from one elevation band to the next, the terrain texture changes.

So... The only way a single sim can have two very different terrain texturing on two halves, would be if one half was very low, and the other half very high, with a steep cliff between the two. The East half might have the two lowest bands available to it, while the West half had the two high bands, simply because all terrain heights n one side or the other were above, or below, the mid transition elevation.

To paint anything less than a full sim with any textures other than what the server provides would require coating the terrain with a phantom sculpted prim "skin" matching the terrain surface exactly, hovering a small fraction above the terrain, and textured differently. That skin of prims wouldn't follow normal terrain texturing rules, and would be extremely difficult to create and maintain.

terrain sculpty with custom textures on them. I done this on a custom sim build.
it went from sci fi to green forrest. took an absolut age to do the floors to look real enough.
and using other megas to stop the falling through the sculpty which need to be phantom.