I would go with the double fans personally. They seem quicker at drawing in and dispersing the air? I don’t have any experience of them, I just have single fans. Are they noisy? 
Because that would be a deciding factor…I’m not keen on the rattling that sometimes happens.
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Can’t make my mind up so I’m going to bottle it and wait for the engineers on here to explain.
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Don’t think they use small fans in wind tunnels. But then again, we don’t use big fans in bathrooms.
Which is why I’m ducking out, 'cos both sides seem to have strong counterarguments.
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Doesn’t that have more to do with the size of the fan matching the size of the space?
In this case, the area of the fan is roughly equal. They’re both roughly 20" across. But in that space, one configuration is one fan and the other is two.
Yeah, but no, but yeah, but no. The velocity of the outer circumference of the smaller blades may (or may not) be faster than the larger blade. But the airflow rate is presumably different depending on how far along the blade one is.
This means something, assuming it doesn’t mean nothing… 
yada yada yada. We need a propeller engineer on here!!
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I can’t do the survey because I have a tower fan which I would recommend as the best.
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I saw those too. Why are they the best?
Equal airflow everywhere along the blade?
Plus you can get them to swing in an arc.
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Something like this?
My issue with that is that it doesn’t fit in the window. At night here, the difference between the inside temperature and outside temperature can be large. I’m trying to get the outside air in and the inside air out. If the fan doesn’t sit in the window, it’s just blowing the hot air in the house around.
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Fans at opposite ends of the house seem to.make sense then, with one drawing in and the other sucking out. Bidirectional would also seem useful, depending on which side of the house is warmer as the day progresses.
Think Bruce may be a useful resource at this point.
Edit - could you add an option to your poll? Something along the lines of “dunno, woild be a guess”
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Thanks. Already doing that with the box fans. Cross wind does help.
One version of the double fans is reversible with one fan going in and the other one going out. The one I was looking at would have to be done manually, meaning turning the whole fan around. I already do this with the box fans.
I’m assuming none of the opinions are professional or certain. But thanks for clarifying your info.
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@butterscotch , Why does an Airbus, ( or Boeing in your case!) only
use one large ducted fan for propulsion and not several smaller fans ??
I will leave you to do the deduction !! 
But it doesn’t. It has one fan (engine?) on each side of the plane, along with smaller ones on different parts of the plane, depending on the plane.
Not sure what this has to do with my fan question.
I’m deducing you don’t want to answer?
@butterscotch ,What sort of airbus are you talking about??
The ones l know only have two turbofan engines for propulsion ?
You must be including the air-con fans ? If so, they are not very efficient ??
Whereas the two turbo-fans are excellent !! 
I’ve 2 fans one cost £340 and the other £54 & a dog
Guess which one really cools the room . Not the dog !
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Correct. Just trying to blow the cobwebs off my maths and physics studies from my youth and apply logic. Doomed to failure, obviously, but it was fun trying to sound competent.
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That’s easy. One doesn’t have blades. Kidding. I bought one of those fans without blades. Used it once. It was on a clearance so I couldn’t return it.
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Winner the floor blade fan £54
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We used frequency fans. They were normally around 70% to 100% of their power rating. Mind you they were in the region of 2,000 kW upwards…
I’ll get me coat.
