Functional Series Representable as a Sum of Two Universal Series

Series with respect to systems Phi = {phi(n) (x)}(n=1)(infinity) of measurable and almost everywhere finite functions are discussed. A necessary and sufficient condition for representing any series with respect to a system Phi as a sum of two universal series is formulated. A consequence of the condition is that any series with respect to an arbitrary complete and orthonormal system Phi is a sum of two universal series.