I’m having fun making a design for a roughly 1.5m X 1m Y footprint laser cutter; big enough to fit a 100W tube in.
At that width, would you put two exhaust fans in, each ¼ of the way in from the ends, so that each is responsible for drawing half the exhaust volume, or would a single fan in the back center be likely enough? I have primarily cutting, not engraving, in mind.
All the bigger laser setups I’ve seen have always used a single big blower. I don’t see any real downside to doing a dual blower setup though, other than maybe cost.
If I build this, I would probably do 90% of the work on a small area in the middle, and anything using the full area would probably be cutting not engraving. My first pass is to do enough design work to estimate cost reasonably accurately. That might cause me to scale down. 
As long as a single exhaust typically works, I’m not planning extra.
I agree with Ned. All the large lasers use a single exhaust. It is the volume of the extraction fan that does the work.
I was thinking 2 smaller exhaust fans might be better if you’re not going to be cutting lots of big stuff. You might have to do an interesting “Y” shapped duct to cover the outer sides of the work area with the main exhaust duct covering the center. This is if you always want to use the center. If you can easily just use the left or right side for smaller work then just split the duct/fans for half and half. Less noise, less power and if it’s hot or cold outside less HVAC air to deal with as you exhaust air outside and it gets replaced with other outside air.
$0.02
Our self-build 150W laser has a 122x122cm bed (half a sheet of plywood here in the Netherlands). We have done one big-ass snail-type exhaust fan that has two points of extraction; one downdraft into the table (if you cut through the material, lots of smoke also gets pushed down and taken by this point. On one side (the right side for us) we have a full-bed-width mesh that has the second exhaust point behind it.
Doing double left/right would create competing are flow I would say. I have also seen people having an active input fan left and extraction on the right so you’ll get a rapid airflow over your workpiece.