It would make sense if both were them. After all, two flans that killed the other were probably mates too. Reimus and Marisas can mate and be viable, but they don't like to do it. Some artists have shown predatory yukkuris as being parasitic in reproduction. Raping a Reimu and then leaving the mother and offspring as food for the young. But them being in a nest means this is a an entirely different scheme. Damn, we need an IRC room.