Here is a solution that works. Basically, you have to REUSE the same item name in the line chart as you do in the bar chart to ensure that you do not introduce a new bar/x-axis point. You do not have to define all four matching points, but to me it seems like that would be good practice. The key to your problem though is to not introduce a new "item" name that results in a new bar. In my example, I just reused "January".
<cfchart format="png" scalefrom="0" scaleto="5000">
<!--- four blue bars --->
<cfchartseries
type="bar"
serieslabel="Website Traffic 2006"
seriescolor="blue">
<cfchartdata item="January" value="1000">
<cfchartdata item="February" value="2000">
<cfchartdata item="March" value="3000">
<cfchartdata item="April" value="4000">
</cfchartseries>
<!--- one red line --->
<cfchartseries
type="line"
seriesColor="red"
paintStyle="plain"
seriesLabel="Contract Salaries">
<cfchartdata item="January" value="2500">
</cfchartseries>
</cfchart>