Hey MPaola,
The documentation for the SG procedures should help a lot. You can also go to the support site and search for "Dan Heath" to find a number of related papers I've written. In the meantime, here is a very simplistic example to demonstrate barline overlays:
[pre]
proc sgplot data=sashelp.class;
y2axis min=0;
vbar age / response=weight;
vbar age / response=height barwidth=0.5;
vline age / response=weight y2axis lineattrs=GraphData1(thickness=2);
vline age / response=height y2axis lineattrs=GraphData2(thickness=2);
run;
[/pre]
Hope this helps,
Dan