Let $\mathbb{F}_q((X^{-1}))$ be the field of formal power series over a finite field $\mathbb{F}_q$. We characterize a pair of roots that lies outside the unit disc while all remaining conjugates have a modulus strictly less than $1$. In particular, we provide a sufficient condition for a pair of formal power series to be a 2-Pisot series. We also give an irreducibility criterion over $\mathbb{F}_q[X]$.