First, you can add multiple line series by a criteria in your Array Collection:
<mx:series>
<mx:LineSeries
yField="Profit"
displayName="Profit"
/>
<mx:LineSeries
yField="Expenses"
displayName="Expenses"
/>
</mx:series>
You can change the width and color of the lines for each series by using the tag. Making each line a different stroke and color.
<?xml version="1.0"?>
<!-- charts/BasicLineStroke.mxml -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Script><![CDATA[
import mx.collections.ArrayCollection;
[Bindable]
public var expenses:ArrayCollection = new ArrayCollection([
{Month:"Jan", Profit:2000, Expenses:1500, Amount:450},
{Month:"Feb", Profit:1000, Expenses:200, Amount:600},
{Month:"Mar", Profit:1500, Expenses:500, Amount:300}
]);
]]></mx:Script>
<mx:Panel title="Line Chart With Strokes">
<mx:LineChart id="myChart"
dataProvider="{expenses}"
showDataTips="true"
>
<mx:horizontalAxis>
<mx:CategoryAxis
dataProvider="{expenses}"
categoryField="Month"
/>
</mx:horizontalAxis>
<mx:series>
<mx:LineSeries
yField="Profit"
displayName="Profit"
>
<mx:lineStroke>
<mx:Stroke
color="0x0099FF"
weight="20"
alpha=".2"
/>
</mx:lineStroke>
</mx:LineSeries>
<mx:LineSeries
yField="Expenses"
displayName="Expenses"
>
<mx:lineStroke>
<mx:Stroke
color="0x0044EB"
weight="20"
alpha=".8"
/>
</mx:lineStroke>
</mx:LineSeries>
</mx:series>
</mx:LineChart>
<mx:Legend dataProvider="{myChart}"/>
</mx:Panel>
</mx:Application>