Schools select special days for dress-up activities, such as pajama day, school spirit day or luau day. On these days, the students dress up according to the selected theme. Twin day is when students pair up and both come to school dressed in the same ensemble. Usually this means a matching outfit but this can be slightly more complex when the pairing is a boy and a girl.
In lieu of simply assembling a matching outfit for the boy and girl, construct a costume for the two to wear to school. Use the idea of two objects that are part of a set to highlight both the twin day as well as showcase the difference between the twins. For example, dress one child as a salt shaker and the other as a pepper shaker. Although the outfits are different, the two are clearly a pair, making it an effective choice for a boy and girl twin day.
Dress the two children according to a theme instead of in matching outfits. A western, sports or animal theme can be easily translated into an ensemble for the "twins." For example, dress the boy as a cowboy and the girl as a cowgirl. You could also dress the boy as a football player and the girl as a cheerleader or the boy as a lion and the girl as a tiger.
Dress the children in outfits that coordinate with each other. For example, dress the boy in overalls with a red shirt underneath and the girl in an overall dress with a red shirt underneath. You could also dress the boy in a dress shirt and tie and the girl in a fancy dress, which will illustrate the coordination between the two and tie their ensembles in with the twin themes.
Clever use of opposites can illustrate a connection between the boy and girl, making the ensembles an appropriate choice for twin day. For example, dress the boy in a white shirt and black pants and the girl in a black shirt and white pants. You can also use patterns to convey connections through opposites. For example, dress the girl in a dress with black polka dots on a white background and the boy in a shirt with white polka dots on a black background.