I would like to have a kick instrument on the Q key and a snare instrument on the W key, to play and record in the pattern.
I know I can do this by having both samples in the same instrument, but this is not good…
Because I need to have the kick and the snare on individual outputs, to apply individual effects and so on.
Is there something I could do to make this possible in Renoise?
I’m not sure if I understand you correctly, but if all you want to do is to use different dsp chains in different samples, you can put your desired samples in one instrument and use a separate track for each sample note with it’s own unique dsp chain.
If, however, you want to use different pitch/vol/filter instrument envelopes on each sample (by having each sample as a separate instrument) and then assign these instrument to different keys on the keyboard, then no, because it’s not possible to use more than one instrument at a time.
It is not possible to control more than one instrument at a time.
But the suggestion is okay.
Use the samples in one instrument, assign them to different keys and then use the instrument upon multiple tracks simultaneously and apply different effects to those tracks using the DSP or VST chains.
With samples inside the RNI structure, this can be done.
You cannot apply different instrument envelop properties that way, but most or even more functionality that is in there, can also be applied on the track using automation and DSP effects.
These DSP and VST parameter values can also be controlled within the pattern through patterncommands whereas envelope properties cannot be influenced by pattern commands.
Thank you, guys!
I didn’t even think of the different tracks… silly me! 
So this basically solves my problem. I can live without the individual instrument envelopes, 'cause I can do some editing in the sample editors…