I would approach each child as an individual so one could be ready more than the other. It can be really enticing to see a sibling excited about wearing underwear and the other child may be excited to participate as well. With that being said, it may also be easier to toilet learn them at the same time. If one twin is picking it up more easily, that child will be a role model for the other. It is definitely better to toilet learn sooner rather than later. The closer to 3 you get, the harder it is. As for the school piece, I have found that pull-ups prolong the toilet learning process because pull-ups are essentially a diaper that you pull up and down. It can be confusing to a child. Plus, children need to reset their muscle memory they have developed from wearing a bulky diaper. A pull up is just as bulky and feels like a regular diaper. They need to feel the wetness and the difference in material that you would get from wearing underwear. What does the preschool do when a child is ready to wear underwear? Are they set up to support a child who is toilet learning? If a child is successfully wearing underwear, are they able to support that or do they still expect the child to arrive in a pull up? If that is the school's policy, there may not be much you can do. If this is the case, then I wouldn't put them into a pull up until right before school and have them change back into underwear as soon as they are out for the day. It seems like extra work but if the school isn't able to support the children who are ready to stay in underwear then you will have to do everything you can outside of school to set them up for success.