You don't need a center channel speaker in a properly-set-up Zu 2 channel installation. On the contrary, a center channel will degrade your imaging and can make dialog less intelligible.
The Definitions acoustically manage the midrange and treble dispersion well, by virtue of the vertical MTM array. They are designed to limit vertical dispersion to reduce ceiling and floor effects, and to broaden horizontal dispersion relative to the single-main-driver Druids. So with Definitions it is very easy to obtain a rock-solid alignment of dialog audio to speaking image, for excellent speech clarity on movies.
With Druids, toe-in is more critical to achieve the same, but when dialed-in the effect is just as stable and focused. The window for ideal sound dimensioning will be somewhat narrower than with the Definitions, but in most situations should be quite satisfying for everyone watching.
In general, the extrapolation of the Zu ideal is that simplicity pays, and that holds true for HT 2 channel as well.
However, for anyone who regardless of this advice insists on center or 5 channel, Tones and Druid Studios can give you the industry-encouraged coverage.
Phil