Plenty of people have fantasies about twins or sisters. It is a pretty common staple of the erotic stories segment. There isn't much of anything special about it, and while it is slightly weird, it isn't out of the range of normal.
Are you sure they are actually doing each other? I know that there is a bunch of stuff side by side, some with other similar looking girls, but I hadn't seen anything where they specifically do each other.
Alex
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