This may be obvious, but I believe the key to a J-P relationship is whether the overall values/goals that each of you hold are similar, or at least mesh well.
Granted there is a bit of obvious generalization here, and a bit too much stereotyping for my usual tastes. But for the sake of extrapolation, I think it'll be okay.
Supposedly J's methods are more constant and sometimes too rigid, while P's are more flexible and sometimes too lax. However, it seems that if your overall goals/values are close, teamwork would be better accommodated and both would be more likely to give a little if they felt the others' strengths were more appropriate for the given scenario. Ie. If one needs to "seize the day" or make things happen, J mindsets have an advantage, while if a situation allows for little-to-no control, P mindsets are much better at coping with that. This would be a pretty balanced relationship.
On the other hand, two Js with contrary goals would be at each others' throats, while two Ps with a similar setup would probably recede from one another too much.
Basically, I don't think types are the most important factor here.
Note: For the record though... I find ENTPs fascinating.