This is something I thought about but the big problem is that DIYers have no way to validate if it's working properly. I mean we're all here because we rely on measurements. Would love to have an affordable commercial product that includes proper crossovers for 2.1.
If you have a signal source (e.g. PC) and a relatively accurate DMM, sure you do. Just calibrate the output voltage coming from your sound card (or equivalent interface), and read the voltage coming out of the xover box while you vary the signal frequency. With a series of measurements you can get enough data points to interpolate the transfer function.
E.g. if you play a 10 kHz sine you should expect full voltage out of the "main" output and no voltage out of the "sub" output. Conversely, at 20 Hz, you should expect the opposite, depending on your filter implementation and how low the xover frequency is.
EDIT: Now, if you were to ask for phase control, well, at that point you should really just consider discrete DSP.