Dear Luluc,
this is a feature normally available in all arranger of a good level.
It can be used in many ways.
Just to make a few example:
-same synth pad sound on R1 and R2, slightly detuned each other (- 4, +4, or more, at your taste), you get a typical majestic pad sound, as used in the analog synths era.
-same guitar sound, as above, you get a sound similar to chorus effect;
-same bright piano sound, as above, you get a honky-tonk piano,
-two different brass sounds, as above, you get more realistic sound, as normally the brass instruments are a little detuned each other.
-and so, on.
Enjoy!
Angelo