I’m aware that this is a pretty old thread now but I have just discovered a quick and effective way of doing this.
I couldn’t justify spending money on EZ Drummer, Perfect Drums etc and could see that Groove Agent has a lot of potential.
Blending drum samples in Groove Agent isn’t that intuative but is actually pretty simple.
In my case I wanted to trigger two Snare samples with one hit on the MIDI Drum Editor.
This will vary on how your drum map is set up but in my case I loaded drum sample 1 on D1 (on the Groove Agent instrument pad) and sample 2 on D#1.
On the Sample 2 on D#1 go to the Pitch tab and activate the ‘Key Range’ button. You will notice that in this case the Low Key and the High Key are both D#1. Simply change your Low Key to ‘D1’ and now the D1 midi note will trigger BOTH D1 and D#1 at the same time.
Now blend your individual drum sample volumes to suit.
Hope this helps.