In Renoise you can use many wav samples or VST plugins.
What I want to do is to have the ability to be able to link two instruments together or more and to be played as one instrument.
For example:
Two Instruments : Piano and Strings
link these two instruments together so I will have a piano-string Instrument 
Then when pressing a note it will play both piano and string together…
Is just an idea 
Cheers…
ages ago i gave similar idea: make in sample button which mix current with other which should play according sample map or also got this button on. somekind of multi-layering. but the devs want to make new sampler in general. the sad thing that we have to wait ages for this one. imho, i’d prefered such button first in 1.3 and then i easy can wait for the new sampler with actual multi-layering.
I also gave a similar idea some month ago…In which you where also able to mix two or more vst’s and samples…Either at the same time or depending on velocity…
probably you can already mix VST instruments in that way.
there are some VST instruments which act as hosts for other VST instruments.
links about them have been already submitted around here, but I can’t remember their names. 