If you purchase compatible units and use thier communication cord or package, they will link together and operate in sync with sequential firing. Yes one will close the inlet valve so that minimum flow to activate one heater is not affected.

Generally in a residential application where you would have enough demand to require two separate tankless you would most likely also have some rather long runs of pipe from the source to the demand. Rather than try to place two units in tandem at one locaton I would be more inclined to divide the loads into two zones and place separate units to handle each zone. That would decrease radiational losses from long pipe runs, help provide a more balanced supply to the demands and would definitely eliminate the need for expensive complex lead/lag control systems.