  Deformable mirrors with very high order correction generally have smaller
dynamic range of motion than what is required to correct seeing over large
aperture telescopes. As a result, systems will need to have an architecture
that employs two deformable mirrors in series, one for the low-order but large
excursion parts of the wavefront and one for the finer and smaller excursion
components. The closed-loop control challenge is to a) keep the overall system
stable, b) avoid the two mirrors using control energy to cancel each other's
correction, c) resolve actuator saturations stably, d) assure that on average
the mirrors are each correcting their assigned region of spatial frequency
space. We present the control architecture and techniques for assuring that it
is linear and stable according to the above criteria. We derived the analytic
forms for stability and performance and show results from simulations and
on-sky testing using the new ShaneAO system on the Lick 3-meter telescope.
