22(Crossover)

An Au where Viper and Skull are the same age and twins(italian mother and russian father) but when they are around 2, their parents divorce and separate them(Viper stays with their mom and Skull with their father).

They live their lives not knowing they have a twin(or any siblings in general) but feel like something is missing.

It isn't until a few years before they meet the Arcobalenos that they end up in TW(one in Night Raven and the other in Royal Sword Academy), that they meet each other during the Neige/Vil thing and feel like they are complete for the first time ever.

They find it confusing but assume it is because their UM complements each other even if their friends say that they might as well be siblings because of how similar they look(they would both he black haired with purple/indigo eyes, they dyed their hair when they return to the KHR world).

Unfortunately as soon as the plot of TW is over, the twins find themselves back in their world and assume TW was just a weird ass dream despite it having been so realistic(and when they find out about flames, they also assume that a mist user may be behind it but never wonder much more about it).

When the meet each other again they feel once again complemented but assume it is because of harmonizing with Luce and do not recognize each other for the first week or so(they would have grown up more, changed their appearances and mostly hide their faces with their cloak/helmet).

Not sure what happens next or how it affects canon but it isn't as Verde is experimenting with their blood to have their adult form back completely that he decides to make a DNA test on all of them to see if matching them with a familiar will help that ti is found out that Viper and Skull are siblings.

Anything else is up to the writer.