It would be nice to have some context for this question.
For example, if you have a fiscal metering station with multiple redundant sensors, you will have voting circuits which compare the readings from each sensor.
The idea is that if you have three density meters they should all read the same but if one of them fails only two will agree.
The voting circuits will use the sensors which agree for the measured value and flag thee fault on the third.
If you only have two sensors, then they have to agree for the readings to be used. In this case I might assume that voting 2 out of 2 infers two sensors in agreement.
Is this something similar to what your question applies to?
In the logic for a shutdown system it is common for the system to include different levels of redundancy. Perhaps a few different a two-out of voting examples exist. Perhaps there are redundent electronic components to read a single I/O sensor Perhaps there are two sensors on different I/O cards. The results of both logic solvers (from the combination of the sensors and electronics) require both components to switch to the trip status to initiate the shutdown. In this example 2oo2 (two out of two) logic would be the least safety protective; and likely eliminates the most nuisance trips as one bas sensor or I/O channel could not cause a trip.
In its simplist form, 2 out of 2 voting means that both inputs/signal/logic must be true for the consequence/result/effect to happen.
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2oo2 is quite limited as a voting system. 2oo3 always gives you at least two switches in agreement which may give you sufficient confidence to go with the majorty vote. 2oo2 leaves you with an indeterminate state where there is a disagreement but you have no basis for assuming which is correct unless you have a third element: the third element could be a transmitter in a hybrid system using two switches plus a transmitter with a trip-amp or a DCS threshold flag.
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