Having girls is probably different than boys. My boys are too young to care but even the older boys don't really care about what they wear. I do buy some matching outfits for fun and for pictures and outings where I want to be able to see them both right away. Otherwise they wear hand-me-downs and right now anything I can find that is clean!
I do try to find outfits where they might be the same pattern but one is in red and the other blue, that type of thing. My boys don't look alike though and I do want them to have their own identities. They will always be the "twins" but they are different kids. So it varies, event to event, day to day.
If I had girls it might be different though. Girls tend to care more about what they wear and there seems to be more competition. Shoot my boys I don't think, would even care if they went out without clothes!
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DS 03, 06 and twins 09!