As an alternative to quantum logics MV algebras have been
suggested ([2], [20]). In a quite general class of MV
algebras there have been constructed sum of any two
observables ([20], [14], [10]). Recently the notion of
product MV algebra has beeen introduced ([17], [4], [5],
[12]). In any weakly $\sigma$-distributive product MV
algebra the joint observable of any observables can be
constructed and on the base also the sum of any observables.
In this paper we show that the two concepts of constructing
of the sum are equivalent.
