Thank you to @cube_girl for alerting me to the existence of these cute Copy/Paste t-shirts for twins. I'm sure they've been around for years, but this is the first I've heard of them:
Riverstone Goods Twins Copy & Paste 2-Pack Short Sleeve Lap T-Shirts (Newborn, 7-14 Pounds)
Almost makes me want to have twins. Or, you know, cats that wear clothes.