My favorite is on one put" I'm a twin " and the other says "Me too"
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.
Take a look at some I had done, with so many freebies it's easy to come up with baby patterns!!
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.
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..
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!!!