Well that's even more confusing. All this time I thought they were the same. But if they were side by side does that mean shared continuity?
According to the series creator (Ryuichi) and also the story in Monster Boy and Cursed Kingdom : yes, same continuity (but separated by centuries I think).
These two pics which supports the claim could fall into spoiler category,
but since the game has been released more than one year I think it's okay if I post these :