Well, you run into the possibility of one yeast being dominant over the other, which is quite likely since they compete for nutrients. At least in theory, you may be wasting money on the liquid yeast if the dry yeast dominates and the liquid yeast does not get a chance to express its characteristics.
That said, the major yeast companies do sell yeast blends that you may be interested in trying out. You can also experiment with stalling a fermentation by cold crashing for an extended period of time, racking to another fermenter (thereby getting rid of most of the original yeast), and then pitching a large, active starter of another yeast in there.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Planned: sLambic II, Flanders Red II
Primary: Berliner Weisse w/ GB 110 & Brett Brux, Berliner Weisse w/ L. brevis, Dark Mild w/ 002, Dark Mild w/ 028, sLambic I
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong
Bottled: Flanders Red I