Telling a shinki that something is not true about their past life will not turn that shinki into an ayakashi. (In other words, "I don't know you" or "We've never met before" would be safe to say, whether it's the truth or not.)

In general, though, gods will feel increasingly uneasy whenever they get close to talking about a shinki's past, and shinki will also feel a sense of unease and potential disaster. So, while a god could say "We're not from the same world" without causing a disaster, both the god and shinki would be deeply uncomfortable addressing a question like "both our worlds have these monsters, so am I from the same place you're from?"