Also, keep in mind that the total weight allowed for the strollers is evenly divided between the seats.
So, a double stroller with a weight limit of 110 pounds will hold 2 children, with each one weighing no more than 55 pounds.
You may want to think about renting 2 strollers. That way, if you need to split up because one child is having a bad day, is tired or can't/won't do what the other child is doing, you can split up and still have a stroller for each of them to ride in.
As was mentioned, look in the disABILITIES FAQs thread near the top of this board or use the link in my signature. Post 2 is about renting mobility equipment. Scroll down toward the bottom for the special needs stroller information.
If the 10 yr old is at least 5 feet tall, he may be comfortable in a wheelchair. One of the companies rents smaller adult ones. If he's smaller than that, I would still opt for a special needs stroller. With low tone, he needs support and would be too slumped over in an adult size wheelchair.
Also, see post 6 of the disABILITIES FAQs thread, which is about Guest Assistance Cards.