Not really examples of inter-religious theological study from beliefs that do not share a common root.
Umm... Christianity and Islam both citing Aristotle or Plato, neither of whom were of either religion, is pretty significant. I suppose you're right; we don't see Christians citing Confucian scholars (well, okay, actually, we did: Nestorian Christianity made a shot at it with some surprising success). But that's probably mostly because they didn't exist side-by-side much.
In every instance where religions existed side-by-side in the Medieval period, cases can be found where they regarded each others' leaders as relevant or even possibly authoritative on spiritual matters. Not necessarily correct, but not simply irrelevant. Not someone that can be written off because we all just have different beliefs about spiritual things and who's to say anyways?
