If you select both characters (either the bottom right icon, right click on an portrait and click 'Select All', or hit tab to being up the party menu wherever the cursor is and click 'Select All'), the should both use their primary attack when you left click on an opponent. For secondary skills (spells, spin attack, etc) only the active character reacts to right clicks (except both will Sneak if either does).
Once you get the DK following you, the demo should be much easier. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />