ON axis is when the speaker is facing you. Like, pointed right at your face. Off axis is any degree of... not that. Could be 30 degrees or 90 degrees or whatever. In a typical door install, the driver's door is usually ~ 60-70* off axis. The passenger one ~ 30* off. This is why when you see FR graphs, they usually give you on axis, 30* and 60*.
The larger the driver, the earlier in the frequencies the driver will have a natural rolloff because of beaming. It's entirely size dependant and not up for debate. 6.5" drivers start to "beam" at around 1.6khz, but not significantly until 2.5khz or so. If your speakers were pointed straight at your face, you might get some interference with frequency overlap at tweeter frequencies. Off axis, this is not likely to be a problem.
As an example... here is my 6.5's in my doors playing full range. The spike you DO see over 2.5khz is just "noise" and was there before I started the test. You can see how little information there is. This is because of beaming.
So, as you can see... you likely don't have to worry about the frequency overlap.
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