There are some quite good youtube tutorials on how to to make splitscreen with paster.
Here is a short version:
1. Create 2 Paster objects with the size of 1/2 screen. Place one on the top side of the window and the other one below it.
Example: Your screen has a 1200/600 pixel rate. Both Paster objects have a size of 1200/300 and their hotspot is in the middle. You now put the first one to position X 600 and Y 150. The 2nd Paster is on X 600 and Y 450.
2. You'll need Families for this to work elegant. Create a Family where you put ALL objects you want to paste. If you have tiled backgrounds, you will need to paste them in seperately BEFORE the Family. Sprite fonts somehow are not supported by this add on, so you can't paste them.
3. Create 2 players.
4. Event Sheet:
Paster clear to RGBA (0,0,0,0) //to clear the screen from the previous frame
Paster set position to Player1.X, Player1.Y
Paster paste Object "AllObjectsFamily" into canvas
Paster set position to X 600, Y 150
Repeat that with Paster2, just change Player1 to Player2 and the end position to the position of your 2nd paster (X 600, Y 450 in my example above)