The "we go together like cookies and milk" shirts have been some of my favorites for the girls. They have a few more that are cute now for girl twins (or siblings).
http://oldnavy.gap.com/browse/produc...scid=774797032
http://oldnavy.gap.com/browse/produc...scid=774798072
http://oldnavy.gap.com/browse/produc...scid=774798112
http://oldnavy.gap.com/browse/produc...scid=758438002
http://oldnavy.gap.com/browse/produc...scid=774798132
We got these for $5 each today: http://oldnavy.gap.com/browse/produc...scid=774798132
(Can't get the link to work, but it's "you're the peanut butter to my jelly"