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Spiders do have two venom-injecting fangs and typically bite with both at the same time. However, in any spider smaller than a tarantula, the entry points of the two fangs will be so close together that there is little if any visible separation. Also, the fangs are so slender and sharp that the actual entry points are all but invisible.
When you have a "bite" with two separated marks, it is either caused by a bloodsucking insect that has bitten twice (a common occurrence), or is a double skin eruption arising from one disease condition or arthropod bite, also a common occurrence."