I did some years ago for twins and can only remember 3 of them:
copy / paste
I was first / No, I was first
Mom loves me more (on both)

I have twin grandgirls all grown up now..but they each had the "I'm the good twin" shirts. and for babies I like "stand back..these cheeks are loaded"....and there's a really sweet twins design with 2 peas in a pod at EmbLibrary. (#C3996) for the smaller size. Also....stitch a bar code design...and put on it BOGO (buy one get one free!!) Love my twins!!!

Not necessarily for twins but I have done lots of them with a design that looks like the gereric peel & stick name tags . It says, "Hi! I'm new here" with sapce underneath for the name. Also cute is use a big initial of the first name and over that in small letters write the entire name. I will add a few links for places you might get some other ideas..

Embroider a whale spewing water and add I Spew or I spew two. Sew forum has a lot of burp ideas and free designs, like Brrrrrrp and add a few bubbles to it. In the search box, type in baby burp sayings and see
if you get any ideas, just keep trying different searches.

Take a look at some I had done, with so many freebies it's easy to come up with baby patterns!!

There are several up to see in the project section. With the ideas here and the ones you can see there you should get a lot of ideas. Have fun.

My favorite is on one put" I'm a twin " and the other says "Me too"