How can I not repeat writing similar code?

I have two very similar functions that is almost a duplicate of each other except that the conditions are opposite to each other. I want to follow the DRY pattern, but I have read somewhere before that passing a boolean into a function to change behavior is a bad practice(since its difficult to maintain). What is my best option here for me to write clean and non repetitive code? Thanks

def find_upper_tangent(left, right):
    l_idx = get_rightmost(left)
    r_idx = get_leftmost(right)
    upper_tangent_left = False
    upper_tangent_right = False
    prev_slope = get_slope(left[l_idx], right[r_idx])
    while not upper_tangent_left or not upper_tangent_right:
        while not upper_tangent_left:
            # move counter clockwise
            next_slope = get_slope(left[(l_idx - 1) % len(left)], right[r_idx])
            if next_slope < prev_slope:
                l_idx = (l_idx - 1) % len(left)
                prev_slope = next_slope
                upper_tangent_right = False
            else:
                upper_tangent_left = True
        while not upper_tangent_right:
            # move clockwise
            next_slope = get_slope(left[l_idx], right[(r_idx + 1) % len(right)])
            if next_slope > prev_slope:
                r_idx = (r_idx + 1) % len(right)
                prev_slope = next_slope
                upper_tangent_left = False
            else:
                upper_tangent_right = True
    return l_idx, r_idx


def find_lower_tangent(left, right):
    l_idx = get_rightmost(left)
    r_idx = get_leftmost(right)
    lower_tangent_left = False
    lower_tangent_right = False
    prev_slope = get_slope(left[l_idx], right[r_idx])
    while not lower_tangent_left or not lower_tangent_right:
        while not lower_tangent_left:
            # move clockwise
            next_slope = get_slope(left[(l_idx + 1) % len(left)], right[r_idx])
            if next_slope > prev_slope:
                l_idx = (l_idx + 1) % len(left)
                prev_slope = next_slope
                lower_tangent_right = False
            else:
                lower_tangent_left = True    
        while not lower_tangent_right:
            # move counter clockwise
            next_slope = get_slope(left[l_idx], right[(r_idx - 1) % len(right)])
            if next_slope < prev_slope:
                r_idx = (r_idx - 1) % len(right)
                prev_slope = next_slope
                lower_tangent_left = False
            else:
                lower_tangent_right = True
    return l_idx, r_idx