Someone posted this site the other day. Lots of cute freebies. On page 3 there are 2 babies looks like they are playing patty cake. Not sure what you are looking for but these are cute.

Sew AZ has a set not free called Baby Signs - it has two sets of booties and says "Our family was just enlarged by 2 feet" or something like that. And I do remember that this site CuteEmbroidery.com had several sets of twin animals - there were free and I bet you could get some votes for one or another of them, if you asked. Why not just create a design with a arrow pointing down - that says "Twins"? And if I remember right - Ricky has a FSL Twin on her site, too.

http://10embroidery.tk/

Go to "downloads" and scroll about 1/4 of the way down.

Artistic Threadworks had a freebie called Benny-and-Boo. It's probably for sale now, I don't know how to find it.
1. here is a page with a sample

not free but these are cute...

Free baby hands and feet at Aunty M Designs.

here are some baby designs. Perhaps you can make them twins by embroidering a design twice: 1 normal and one in mirror image so it becomes twins.

Here are some twins. You need to go to Yr.2001 and scroll down to baby then just click on the picture.

ttp://www.kreatieve-brievenbus.nl/koop/index-koop-nl.html