You should instantiate them as follow:
var aGrid = new Ext.grid.GridPanel({
bbar : pagingToolbar1
});
var bGrid = new Ext.grid.GridPanel({
bbar : pagingToolbar2
});
Another way that I've been using is:
var aGrid = new Ext.grid.GridPanel({
bbar : { xtype: 'pagingtoolbar', store: 'Ext.data.Store'}
});
var bGrid = new Ext.grid.GridPanel({
bbar :{ xtype: 'pagingtoolbar', store: 'Ext.data.Store'}
});
Using two instance of each object (such as store or pagingtoolbar) is absolutely necessary.