You only pay 1 less for a spell with both colors. It's only a single ability that checks whether a card meets its conditions. It either reduces a spell's cost by 1 if it is red and/or blue or it does nothing. If it had 2 separate abilities, it would be another story.
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No. Nightscape Familiar's cost-reducing ability is still just a single ability, so it applies only once even if the spell has both colors.
Contrast with Grand Arbiter Augustin IV, who has two separated cost-reduing abilities, so if you cast a white-and-blue spell it will cost 2 less.