Of course, it's the old math theory trick. If two cards pointed 0 and 180 respectively are basically an Omni, what would two Hypers facing 0 and 180 act like? a figure 8 with a mini bulb in the middle of the pattern where they meet at the rear of each cap? I'm not 100% certain here.
Two back to back coincident cardioids sum electronically at an equal ratio produce an omni. Two back to back coincident hypercardioids summed electronically at an equal ratio produce..... an omni! (with lower sensitivity).
Lets walk through it- Think of the pickup pattern of a microphone as mathematical combination of an omni component and a bidirectional (figure-8) component mixed together at a particular ratio. That ratio determines the pickup pattern. An omnidirectional microphone is 100% omni component and 0% bidirectional component. A cardioid is equal amounts omni and bidirectional. The hyper is less omni and more bidirectional. A figure-8 is all bidirectional and no omni. Good so far?
A bidirectional mic has a specific orientation polarity. It's front side produces a positive polarity signal and back side a negative polarity signal. An omni has no specific orientation polarity. Summing any two opposite facing
coincident mics, the omni components double, and the bidirectional components cancel out (positive lobe + negative lobe | negative lobe + positive lobe). If the mics were to face the same direction, the omni components would still double, and the bidirectional components would double as well (positive + positive | negative + negative).
So with two summed back to back cardioids, the bidirectional components cancel, and the omni components double. Given the 50/50 ratio of omni/bidirectional components in a cardioid, we're left with a doubling of the two 50% omni components, yielding 100%, or equal sensitivity to one of the single cardioids we started with.
With two summed back to back hypercardioids, the bidirectional components cancel, and the omni components double like in the previous example. But given that a hypercardiod is made up of a higher ratio of bidirectional component to omni component, we're left with less omni component to add together, yielding less than 100% of the original sensitivity compared to that of the single hypercardioid we started with.
Summing a pair of coincident but opposite facing bi-directional mics leaves nothing, as there is no omni component provided from either microphone. If we want to combine opposite facing coincident directional mics to produce a figure-8 instead of an omni, we need to polarity invert the signal from of one of the microphones. Then the bidirectional components double and the omni components cancel. Two summed coincident yet opposite facing directional mics produce a pseudo figure-8, and two summed coincident omnis leaves nothing.
This is how the signals from two opposed cardioid capsules inside in a multi-pattern electrically switched microphone can be combined in various ways to produce any pickup pattern from omni through cardioid through bidirectional.
That's the mathematics of it at least. In the real world, it's never that clean and neat. Capsules arranged so as to be as coincident as possible won't cancel entirely even if levels and responses are precisely matched, and if we introduce any significant spacing between the mics then the resulting time/phase interactions insure that none of the clean theoretical mathematical stuff above holds true anymore.