Two fans in series is normally used to provide adequate (whatever is decided by whomever is designing it) flow in a relatively high resistance application, like ductwork. Each fan moves the same amount of air, of course, but operates higher on its flow rate vs static pressure curve.
It's also used where you want a neutral pressure in between the fans, but that's usually a consideration where you're moving lots of air (like a building) and the thing that could be negatively affected is relatively small, like a gas or oil burning device where you don't want to pull or push air through a chimney with the fans. Or the doors of a store where you don't want to pull a vacuum that people struggle to overcome.
But in an engine room, just putting adequate ventilation intake area (taking into account the effective reduction of that area by the louvers, which can be substantial) is cheaper but requires lots more area (loosely speaking) than forcing intake through a smaller area with a fan or blower.
So it could be that the designers didn't want to use large intake grills, or there's intake/exhaust ductwork, or whatever. You'll almost always move more air with both blowing out, as long as there is low static pressure across the intake grill or through the intake ductwork. Which I'm guessing is the reason for having intake fans vs just large intake openings - they didn't want large openings.
Those little solar vent fans, for what they're worth, can be reversed for intake or exhaust, but they move so little air, you'd be better off using two exhaust with the intake air leaking in from everywhere, rather than one in and one out. But higher air flows using real fans or blowers may require boosting on both ends, if lots of intake duct/grill area isn't feasible.
I'd be curious to know how much of a vacuum is pulled in the typical engine room with it/them at speed, with only the normal vent openings working, with or without fans running. Or with intake fans only running, assisting in bringing in combustion air, and air also drawn in backwards through the exhaust fans. Or with exhaust running, pulling a vacuum against what's already a vacuum from engine induction air.