I think you can do it passively, but the results depend on how well matched the pickups are - it may work fine, but more likely the P will over power the J and you may also get some loss of midrange from frequency cancellation in the combined ranges. I'd be tempted to go VVT in a passive set up, or even concentric VT for each pickup (like a stacknob J).
The best I've used was a Sadowsky PJ. Although it was active, I don't think the blending was part of the circuit. But it did use custom DiMarzio humbuckers for both pickups, so I guess they were well matched to work together.