The water might be a little too lacking in tannins - in my experience tinc tadpoles don't do well in water that doesn't have the chemicals from old leaves.

That aside, I am surprised you haven't found eggs if you truly have a pair. I suspect that you have either 2 males or 2 females. I have 2 adult male Citronella who grew up together and they never fight or show serious aggression. So it's possible to keep 2 males together - that might be what you have.