PhlyingRPH made a very good comment on piggybacking a briefcase on a rollaboard here:
http://www.flyertalk.com/forum/12995363-post7.html

There are two ways to do that. One is to stack the briefcase atop the rolling luggage and attach it to the expansion handles. This is quick and gives a flat and high silhouette. The other option is to hang the briefcase from a hook resting against the front side of the roller. This is also quick except that you need to affix the hook. It will give a bulkier look but has one potentially important advantage. The center of gravity will be lower and the weight will be born by the wheels instead of your forearm and shoulder.

Try it out. If you have a heavy briefcase this really makes a difference.

Thanks, PhlyingRPH!