If you want to assign the value 2 to the grid at the cell which is active from within the renderer class, you can try this.Grid.Model[this.RowIndex, this.ColIndex].CellValue = 2;.
The grid would not care whether the value 2 came from one table or the other.
Normally, you override renderer.OnSaveChanges to save the value from the control in the renderer to the Grid. Here is what the GridCellRendererBase.OnSaveChanges looks like.
protected virtual bool OnSaveChanges()
{
TraceUtil.TraceCurrentMethodInfoIf(Switches.CellRenderer.TraceVerbose, this.ControlText);
if (CurrentCell.IsModified)
{
Grid.Model[currentRowIndex, currentColIndex].FormattedText = this.ControlText;
return true;
}
return false;
}