This would work... http://jsfiddle.net/WRTM3/1/
Ext.define('myKindOfStore');
var store1 = Ext.create('myKindOfStore');
var store2 = Ext.create('myKindOfStore');
It seems that the grouping feature is actually executed on the store behind the grid. Since you use the same store in both grids each grouping plugin act like two different light switches controling the same bulb...
If you want to seperate grids behaviour, just define a new store and create two seperate store instances for each grid (see my jsfiddle fork)
This way all operations on grid1=store1 have no effects on grid2=store2 including filtering, sorting, etc.
Hope this is what you wanted :-)