If your VSTI is a synth VSTI, this probably won’t work out.
For sampler VST plugins this will work out.
Beware though that the audio from all Aliases are being equally treated as one audio-stream even if you would split these instruments across several physical tracks in Renoise, all different effects are applied to all instruments.
wait, did you mean that you added 1 vst instrument for bass and then added totally separately second instance of same vsti for lead and both instruments playd same sound??? thats very strange…