Unless you have some special constraints I see no advantage to multiple exhaust blowers. With that high of an exhaust rate I suggest considering a single commercial blower with a variable speed motor and a matching makeup air unit.

Based on my somewhat limited experience, I would go with two. Apparently commonly used electronic speed controls by design cannot go below about 40% of full tilt. Reasoning is if you turn down a unit to say 20%, then have a power failure there will not be enough electricity applied to the blower to start it spinning again when power is restored. It will be in a stall condition which will lead to overheating and potentially a fire (this is why most variable speed fans, like a window fan, start on high rather than low when turned on; they need full ac power to overcome the rotational inertia of the motor rotor and fan blades) So with one blower the lowest you could go would be 1000 cfm which will be noisy. Two blowers would let you turn one off completely and have a low speed of about 500 cfm-not exactly silent but better. Also lets you apply ventilation over only the part of your alter that needs it.

Hmm - that's what I was thinking. Basically, ~1000 CFM per side of the cooktop.

It's not clear to me if any hood can be configured this way. I've been looking at the Broan Rangemaster. Can it be configured with two blowers?

Also, are all blowers equal? I have a 1400 CFM Thermador now (roof mounted) and it's very loud.

The blowers I am familiar with are controlled by variable frequency drives (VFD). These speed controllers are designed to allow a motor to deliver high torque at variable speeds. There is no danger starting a motor set to a low speed. When a VFD starts a motor, it initially applies a low frequency and voltage to the motor. Starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply turning on a switch. When a VFD starts, the applied frequency and voltage are increased at a controlled rate to accelerate the load without drawing excessive current. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 50% current.