I am an identical twin, and I had my tonsils out when I was in fourth grade. I don't think my sis wanted to have the surgery, but I am pretty sure she was jealous about all the things I got afterward. I got ice cream, bedrest, presents from one of my mom's friends - one for each day of the week I recovered.
Is there a way you can give special attention to your twin not going in to surgery too? Also remind him that it's not all fun and games, his brother will be feeling under the weather. Maybe he says he wants surgery too because he's afraid of being separated?
Maybe my sister will hop in here because really she'd be the one better to know how it feels.
Mama to my spirited J, and L, my homebirth: baby especially DTaP, MMR (family vax injuries)