I can see myself using both of these, if I am to begin with two characters:

- One of the twins is younger sibling to one of yours, peppy and encouraging.

- One of the twins is a childhood friend of yours but instead of being a retainer of sorts, they joined the army to become a knight and have recently reunited to bring the issue to light.

Brings me back to C&C with the first one; has more of a commoner connection  As for the second, could be related more to a character of royalty.

In regards to the curse. Does it only take effect if revealed by spoken words?  Can they speak of the demon on the lost world or is it only possible to show what has already taken place? Once I figure out that the terms of the curse, I have no problems with the premise. I think if we have our characters taken there prior would be better.   

As for a Yokai Watch 1x1, you do still tempt me.  I selected otherwise for the benefit of our group RP.  The idea remains an open possibility at any time, if really wished for.