Oh, sure, you can hear comb-filtering from a two-driver system. If you put it on the floor or suspend it a few inches from the ceiling with no elevation tilt, your ears will be outside the optimal plane where the difference in the distances from both drivers to your ears are insignificant. But if the speakers are precisely at ear level, then you won't hear any destructive interference in the direct radiation from the drivers. This may be more practical to achieve if you sit far from the speakers -- if you've simply got them on your desktop for PC use, the vertical sweet spot will be small.
One thing that may be quite a bit more audible, however, is a decrease in audible reverberation from hard flooring and bare ceilings. Essentially, arranging the two drivers in the vertical plane will create vertical pattern control. If you sit with your ears aligned horizontally between the two drivers, as frequencies increase, you'll notice more beaming in the vertical plane. The vertical polar (lobing) diagram at high frequencies will feature a narrow point on the horizontal axis, with additional points above and below the long beam as the drivers go in and out of phase at the frequency for which dispersion is being plotted. With the listener's ears aligned with a point equidistant between the acoustic centers of the two drivers, the listener would not be sitting in a null, but in the area of strongest HF radiation.
Quite a few makers of single-driver loudspeakers have had success with dual-driver designs where the two drivers were front-firing. Omega and Almarro are among them.
