I wouldn't stress over it, they only get to walk down the aisle together for about 30 seconds. You could let them stand together in pictures though, and that would be really cute! Also you could make them feel special by making sure the photographer gets a picture of just them together. My fiance and I were in his brother's wedding but he was the best man so we obviously didn't get to walk together. It really didn't bother me because we still got to look at each other during the ceremony. It was cute because I didn't see him all day until the ceremony so it was still special even though we weren't the ones getting married!

Hope that helps a little.