Phasing a biamp system

I currently am biamping my system. The speakers I’m using are not the issue. What would you recommend the best way to be sure the high end is in the same phase as the low end. Can’t really rely on cable polarity since we don’t know what’s going on between the input to the speaker and the speaker.
Two ideas come to mind:
1) use a dual trace oscope to compare the inputs to both the high and low speaker inputs and observe the waveforms of a suitable frequency.
2) use microphone input to the oscope to do the same.

any thoughts or suggestions are very welcome.
thank you