The stability of a negatively charged polyethylene-structured [GaH2](n)(n-) cluster ion was investigated by varying the K+/Rb+ ratio in (KxRb1-x)(n)[GaH2](n) (0 <= x <= 1). Neutron, X-ray, and IR spectroscopies were used to characterize the new phases. Between the limiting compositions K-n[GaH2](n) and Rb-n[GaH2](n), the [GaH2](n)(n-) chains remained almost identical, indicating a stable specie. For rubidium-rich samples up to a potassium content corresponding to (K0.5Rb0.5)(n)[GaH2](n), two phases coexist in the samples, RbGaH2 and (K0.5Rb0.5)(n)[GaH2](n), with a ratio mirroring the relative alkali-ion content. The two phases have a different alignment of the [GaH2](n)(n-) chains. For potassium-rich samples beyond (K0.5Rb0.5)(n)[GaH2](n), the samples were single-phased. The unit cell volume of the new (K0.5Rb0.5)(n)[GaH2](n) structure type shrinks according to Vegard's law as smaller K+ ions substitute for larger Rb+ ions. The [GaH2](n)(n-) chains remained, however, almost identical. IR spectra from the different phases were very similar, exhibited stretching, scissoring, and rocking modes similar to those in ordinary polyethylene, but shifted to lower frequencies, reflecting weaker Ga-H bonds. The existence of stable K-n[GaH2](n) and Rb-n[GaH2](n), would help to dehydrogenate KGaH4 and RbGaH4 upon heating. If this could be transferred to lighter alanates and borohydrides, it could be possible to develop more functional hydrogen-storage systems.