Two ovens on one breaker: NEC says OK, but installation manual wants dedicated circuit
You can normally connect 2 ovens on 1 breaker per the NEC but the manufacturer instructions over ride the NEC per 110.4.B ,
 listed or labeled equipment shall be installed in accordance with any instructions included in the listing or labeling.
 This means you have to follow the manufacturers instructions.  I agree with @JReaf that the control cycling of the elements in one oven may damage the controls in the second oven. We see this in industrial plants all the time and put spike suppressors (MOV’s or diodes on relays depends on if ac or dc) control.  But with the mfg instructions stating they need to be on separate breakers a code compliant install requires 2.