On sorted data, two indexes, one at each end, can often find an answer in one pass. To find a pair adding up to a target in a sorted list: if the pair is too small, move the left pointer right (to a bigger number); if too big, move the right pointer left.

def pair_sum_sorted(nums, target):
    left, right = 0, len(nums) - 1
    while left < right:
        total = nums[left] + nums[right]
        if total == target:
            return (nums[left], nums[right])
        if total < target:
            left += 1
        else:
            right -= 1
    return None

print(pair_sum_sorted([1, 3, 4, 6, 8, 11], 10))

Each step rules out one value for good, so it's O(n) with no extra memory.