Ditto Amargith, NFP/STP usually seems easier than NFP/NTP... my brother is ISTP and it's like she said about recognition of very different thought processes. We rarely think similarly, but we often end up at the same conclusion, and it's useful to have each others' perspectives. I think it would also relate to how heavy the dominants are. If one is super-Ti and one is super-Fi there will probably be more disagreement.
But I think there's also a certain degree of understanding in any IxxP / IxxJ / ExxP / ExxJ pair. All IxxPs will generally share a preference for calm, independence, and low drama.