Okay, having just played Devil Summoner: Raidou Kuzunoha vs. the Soulless Army, I just had the weirdest idea.
Sakura Taisen. I have no clue on how well these would interact, especialy as I've only read a volume or two of the manga.
But the time period seems he same at least, which is about what? 1920ish? They both fight demons, they're both spiritualy aware, the big difference is the method. One uses mecha, the other demons. No clue where to take this beyond the basic premise, but I know that there are games behind the Sakura Taisen series, and figured that it'd be best to ask people who are knowledgeable about it, before coming up with something that wouldn't work.
EDIT of clarification +2: I'm proposing a cross between Shin Megami Tensei: Devil Summoner: Raidou Kuzunoha vs. The Soulless Army and Sakura Taisen.