>> first step was fxbus 2/3 (synthl/r) to asio 4/5, but cubase recorded only the right channel.
>> i've tried this, because fxbus 2/3 are the default synth outs.
>> fxbus channels are mono channels, how can i record stereo samples (gm drums etc.)
synth is stereo (fxbus 2/3) and there should be no problem recording it in 'stereo'
(that is, each note is panned between fxbus2 and fxbus3)
try different asio inputs (especially if you have a '5.1' live board), for instance, asio8,9 ('In 5 L/R')
make sure you pan your synth correctly via MIDI controllers (should be ok by default)

make sure you record in stereo (in cubase)
/Eugene