bro, you need a 2 to 1 ratio for a passive intake system, 2 times bigger exhaust than your intake. the exhaust fan will suck in the air. did some searching on this, check this out.
use only exhaust fans at the top of the cab where the air is hottest with passive intake hole(s) near the bottom (best to use several small intake holes spread out evenly along the bottom)
here's the first and most obvious reason as to why not to split 2/2
If you are pulling air out the top with 2 fans at 150 CFM, that air *has* to come from somewhere, it will pull it through your passive intakes.
What would adding 150 cfm of in-take air actually do? if you are pushing air in at the same rate you are pulling it out anyway's, you haven't actually gained anything. the exhaust fans might not have to work fractionally as hard, but not a noticeable amount. Think about it like this... if you set the cruise control in your car at 10 mph, and then somebody starts pushing your car from behind at 10 mph, your still going to be only going 10 mph, you might just burn a little less gas in the engine because some of the work is now being done by something else.
The other reason I can think of is that, in general, having a slightly negative air pressure inside the box (you will have negative pressure because the fans will be constantly pulling air through the intakes trying to get them to keep up with the rate its pulling it out, it won't be able to do this perfectly, which will create slightly negative pressure). This is a good thing because negative air pressure = less air = less heat capacity which means the hot air will rise to the top quicker and ultimately get exhausted quicker.
the other issue with using combo intake/out take is that you can create sort of channels of air flow that go directly between the intake fan straight to the out take. This happens because the air coming in is being pushed into a certain direction at relatively high velocity. ultimately this can create spaces in the box that doesn't get its air exchanged frequently and becomes "stale" so-to-speak... sorry I know this particular point probably makes absolutely no sense... I can completely visualize it in my head, but I just can't seem to put it into words lol
the best thing IMHO, put all your exhaust fans spaced out evenly across the ceiling of the box if possible. then drill many very small holes (in comparison to the out-take holes) all the way around the perimeter of the box near the floor of it, this should help keep air being exchanged throughout the entire box.
If you want to get like super awesome... you could build like a raised floor inside the box and have all the in-take holes in the raised floor, that would keep fresh air coming directly up through the floor, past the plants, up to the lights and eventually out the exhaust. I actually really wanted to do that when I built my box, but lazy ness got the best of me!