where N > 2s, α, β > 1, α + β = 2N/(N − 2s), and f, g are nonnegative functionals in the dual space of Ḣs (R N ), i.e., ( Ḣs ) ′ f, u Ḣs ≥ 0, whenever u is a nonnegative function in Ḣs (R N ). When f = 0 = g, we show that the ground state solution of (S) is unique. On the other hand, when f and g are nontrivial nonnegative functionals with ker(f )=ker(g), then we establish the existence of at least two different positive solutions of (S) provided that f ( Ḣs ) ′ and g ( Ḣs ) ′ are small enough. Moreover, we also provide a global compactness result, which gives a complete description of the Palais-Smale sequences of the above system.