Just call addSeries putting the other data of line graph.
See my implementation:
GraphViewSeries line1Series = new GraphViewSeries("LINE ONE", new GraphViewSeriesStyle(Color.GREEN, 3), new GraphViewData[] { new GraphViewData(0, 0) }); // put an complete array
GraphViewSeries line2Series = new GraphViewSeries("LINE TWO", new GraphViewSeriesStyle(Color.BLUE, 3), new GraphViewData[] { new GraphViewData(0, 0) }); // put an complete array
graphView = new LineGraphView(this, "Graph with 2 lines");
graphView.addSeries(line1Series);
graphView.addSeries(line2Series);
graphView.setShowLegend(true);
graphView.setLegendAlign(LegendAlign.TOP);
graphView.setGraphViewStyle(new GraphViewStyle(Color.DKGRAY, Color.DKGRAY, Color.LTGRAY));
graphView.redrawAll();
I hope this can help you.