In this Letter we propose a closed-form expression for the average bit error rate of intensity modulation/direct detection free-space optical systems employing dual-branch equal-gain combining and operating over turbulence channels. To offer a realistic error performance of the considered system we assume that both branches undergo independent but not necessarily identically distributed gamma-gamma fading. Our newly developed formula is obtained in terms of the bivariate H-Fox function, which can be readily evaluated based on its two-fold Mellin-Barnes representation. Numerically evaluated and computer simulation results are presented that verify the accuracy of the proposed mathematical analysis.