def choose_positions_to_merge(main_list, position_shift):
    """
    >>> choose_positions_to_merge([0 ,1, 2, 5, 8, 9, 10, 11, 12, 20, 33, 34], 1)
    ([0, 2, 5, 8, 10, 12, 20, 33], [1, 9, 11, 34])

    >>> choose_positions_to_merge([0], 1)
    ([0], [])
    """
    result_main = []
    result_disappearing = []
    i = 0
    while i < len(main_list) - 1:
        result_main.append(main_list[i])
        if main_list[i] + position_shift == main_list[i + 1]:
            result_disappearing.append(main_list[i + 1])
            i += 1
        i += 1
    if i == len(main_list) - 1:
        result_main.append(main_list[i])
    return result_main, result_disappearing
