There are two ‘Stereo’ input channels available in GLD-80, you could use those for your keyboards: locate ch#45 or #47, select one and go to the preamp page, select the sockets where your keys (DI boxes?) are plugged in and apply. you now have one fader, one gain for two channels of synth. Park the fader wherever you need it on the surface,
(if you still need ‘stereo’ channels for iPhone, CD etc use ganging or DCA as suggested above).
please do not adjust your set; normal service will be resumed as soon as possible!