Never assume your solitary child is unhappy about being solitary. I have never, ever disliked being solitary. It is something I look forward to and plan for during my day. I cannot get through the day without having spent some time by myself.
When I was a child, my sister and I spent a lot of time in conflict because she wanted to play and I didn’t. I did play with her sometimes, but most of the time I wanted her to leave me alone. This caused arguments because she was very insistent, three years younger than me, probably bored and wanted company, forcefully interrupted my solitary game without my permission or agreement or desire, and then complained and whined about me to our Mum. Then came the screeching and the reprimands and the punishments and the tears and the destruction of my activity, the ruination of my plans for the day – ie change in planned routine – and then crying and more crying, and me ending up hating everyone, when if they had just left me alone in the first place, the whole house would have been harmonious.
Your ASD child is not like you. They probably like being alone. Let them get on with it and don’t feel sorry for them. Don’t throw another child into the mix in order to occupy the non-ASD child, or to force a lesson in socialisation on the ASD child because you think it’s best. It IS good for an ASD child to learn social skills, but warn them in advance. For example: “You can play that now but later/tomorrow, you will be spending time with your sibling/friend/me etc because it’s good for you/a nice change/good for the other person”.
I used to have a mental plan of my day including everything I thought would be happening: school, meals, chores, bike ride, TV show, whatever. If that gets messed with even today, I’ll get upset. Nowadays I can handle a change in routine merely with a bunch of grumbling, tightened diaphram and deep breathing. When I was a child, tantrums … meltdowns … you know the rest.