Can the two "high" input to a digital logic gate have different voltage values?
Read the datasheet.

There will be a spec for the minimum input voltage so that the signal is guaranteed to be interpreted as a logic high.  As long as the two inputs meet this criterion, and don't violate any other spec, the logic gate will see both its inputs as high.  There is generally no requirement that the two signals be within some voltage of each other, only that each be a valid logic high voltage.

Yes, this really is as simple as it sounds.