I am in the process of building a set of 4pi speakers. I bought a pair of JBL 4530 scoops loaded with 2226H woofers off of Marketplace. They could be in better condition, but should be well broken in. I found new B&C de250s and some other things like grill cloth from Q-components. The crossover parts I found from Solen. Except the 100w 8ohm resistor, I had to order that from Parts Express. I ordered the wave guides from Wayne. Shipping and the broker fees were expensive to Canada.
I have been making slow but steady progress. The crossovers are assembled and soldered. I ran out of black zip ties and had to use white ones for the second crossover, my OCD might take over and I will probably change the first crossover to match. The cabinets are mostly put together. My little router did not have fun cutting out the 1½" baffle, it definitely payed for itself so far on this project. I need to make the port tubes and cut some braces. Seal the boxes. Figure out grills. Pick out some venner, and get them completed.
1. Overall response curve
2. Low-frequency content presented to the tweeter
As an academic study, you can measure the inductance of two coils placed in various positions and you can measure the amount of mutual inductance from the coils and signal coupling as a transformer effect.
But if you just want to see how your crossover layout effects the performance of your loudspeaker, focus on the two things I described above. We just want to know the basic transfer functions of the filters, and the things we're most concerned with are making sure the tweeter doesn't get too much low-frequency energy and that the overall response curve is right.
I say this because I had similar worries when laying out my crossover board. I could put the coils at opposite ends of the board, but then the traces were longer and not as organized. Or I could layout the physical circuit similarly to the schematic, making a clean and organized board but putting the coils closer together.
So I used measurements to drive my decision, and I tried various combinations of positions and orientations that kept the transfer functions right.