They will fight each other and setup an oscillation between the pair. If you put them one after the other the second pulls air from the first and motor electrics don't like being pulled faster. One time where two isn't better than one. You are welcome to try though. :wink:
The logical way to make two extractors work is to split the system.
One each end of the workshop with no join between the two. That way you halve the run of ducting which gives greatly improved air flow.
If you have one machine that produces excess waste (thicknesser) have that side of the run shorter than the other.
Or split your machines into those that generate lots of dust....table saw, sander, linisher and those that generate chips ..spindle moulder, P/T. Then use two different types of dust extraction. Lots of people will take those chips because they are relatively dust free.
If you havent already got the small ones, absolutely buy bigger, MUCH BIGGER.
Dont skimp on it. Your machinery can always grow into the extraction system, but if you go small and then buy more machines, youre stuffed.