Can you explain that again to me in stupid language please
Two fans in series will increase pressure but not air flow.
Two fans in parallel will increase air flow but not pressure.
In the real world, these things interact.
An analogy might be blowing through a straw - the amount of air one can get down the straw will vary depending on the size of the straw and how hard one blows.
It's pretty easy to blow gently through a straw of any size, but it becomes progessively harder to push more air down it as one blows harder and the back pressure increases, and it eventually reaches a point where one simply can't blow any harder.
Now imgaine that the straw is Y-shaped, with two people blowing into it - At first, it will be easy for both of them to blow, and the air flow out of the straw will be more than that of a single person blowing at the same rate. But when the back pressure reaches the point where one person blowing can't overcome it then the flow rate will be no better than with one person blowing as hard as they can. This is like fans in parallel (Y-piece).
Now imagine the absurd situation of a person in a sealed box blowing through a straw that passes out of the box, and another person blowing air into the box. The person in the box will be under a slightly greater atmospheric pressure than normal, so when they blow through their straw they'll be doing so with the pressure they can create plus the pressure that the person outside is adding. This is like fans in series.
Intuitively, with a large straw one doesn't need much pressure to move a decent amount of air, so putting fans in parallel is perhaps a better idea, but with a smaller straw it might be better to have them in series to generate some extra pressure.
If you had matching-sized fans, duct, and filter, and especially if the duct were of any significant length, then I reckon that putting the fans in series might be best, to overcome the back pressure. But, the news that you have an oversize filter (larger straw) does complicate things a bit because it will be able to pass more air than the smaller one (skinny straw) at the same pressure, so any increase in pressure gained by putting fans in series might not be so beneficial, perhaps even not making any difference at all if the fan is already operating at close to its maximum flow rate.