Where a trade or profession is carried on by two or more persons jointly, the liability of any one of them in respect of Class 4 contributions shall arise in respect of his share of the profits F2... of that trade or profession (so far as immediately derived by him from carrying it on); and for this purpose his share shall be aggregated with his share of the profits F2... of any other trade, profession or vocation (so far as immediately derived by him from carrying it on or exercising it).

Where sub-paragraph (1) above applies, the Class 4 contributions for which a person is liable in respect of the profits F2... of the trade or profession carried on jointly (aggregated, where appropriate, as mentioned in that sub-paragraph) F1shall be charged on him separately.