This paper considers stability analysis of uncertain and time-varying systems containing harmonic oscillations of the form cosine and sine simultaneously. The analysis is performed by investigating the stability of a feedback interconnection through Integral Quadratic Constraints (IQC) approach. A new class of IQC multipliers, derived in frequency domain, is proposed to deal with simultaneous cosine and sine harmonic oscillations. The proposed multiplier reduces the conservatism with respect to other multipliers by exploiting frequency and phase shift information. A multiplier parameterization admitting real state-space realization is proposed. Such a parameterization allows to perform stability analysis through a finite-dimensional LMI optimization problem with a reduced number of decision variables.