How can I accurately find the distance between two drones over moderate distances?
At about 1 foot per nanosecond, you might use fast risetime LASER pulses, off corner_reflectors. This is how we precision_range the moon, using  several square feet of optical reflectors put in place during Apollo.
You might also use swept_frequency radio waves, such as what planes use to determine altitude. You'd need a repeater on the collaborating drone. The phase shift sets the distance. Calibration of baseline delays would be needed.
The 800 and 900 MHZ ISM bands might work and might be legal.