The same goes for this situation when ordering a Pimp Flexible design. You can order the splashback on the total dimensions of the back wall and cut it to size in the corners. So you can add the sizes together to create one order.

If you need to do a lot of cutting it might be a good idea to add a small margin to the order (in case you make an incorrect cut or the corners are not 100% straight). Again, place a remark on the order form if you want us to take it into account when creating the design for your splashback.