By using the Mid/Side technique. The output of the Aston Spirit set to figure 8 pattern, and set up so this 8 happens left and right from the cardioid pattern of the Origin. Then, the Origin is recorded in mono (as in the same signal left and right) while the output of the Spirit is split and phase reversed on one side. This way, the phase offset happening between the Origin and the Spirit create the stereo image you hear. The nice thing about this is how it's very monocompatible and how you can just combine different types of microphones if you fancy, as long as there is a cardioid in the center and a polar 8 pattern from another microphone in parallel happening. Also, read up about Mid/Side processing, it's an interesting world of its own. :)

You can't boost what isn't there. I'd move the mic or rely on other instruments (string bass) to fill in the gap.

If the intention were to make a solo cello recording the Spirit does not perform well at all, in a larger mix it could definitely work if say it were supported by an instrument in a lower octave. But yeah, I doubt these musicians and engineer were that impressed.