Easy as pie. Manually:

1. Click on title, wait, click on title  again -- not a double-click
2. Move the cursor to the end of your first line  and hit enter
3. Highlight the first line and increase the font
4. Escape out and you're done

Using VBA, something like this:

ActiveSheet.ChartObjects("Chart 1").Activate
ActiveChart.ChartTitle.Select
Selection.Characters.Text = "Line 1" & Chr(10) & "Line 2"
Selection.Characters(Start:=1, Length:=6).Font.Size = 20
Selection.Characters(Start:=8, Length:=6).Font.Size = 10