2nd fans usually make little difference, but it depends on how good the single fan is. Low pressure fans benefit from being stacked in push/pull on a tower. This is because the static pressure rating is increased by stacking with a flow staightener between them (the cooler) This means a 1500rpm fan ratied at 60cfm and 0.8mmH2O static pressure (static pressure is maximum pressure fan reaches when it's airflow stops moving). When fan slows down it bullds less pressure .. so at 1000rpm will have a static pressure rating of maybe 0.5mmH2O. Now if cooler has lots of resistance (like about 0.4mm H2O) the fan is only 0.1mm H20 stronger than resistance, so is moving very little air though it. When we go push/pull the pressure rating goes up to by about 0.4mm H2O and we now have 0.8mm H2O static pressured rating against the 0.4mm H2O of cooler and get more air movement.
But if the fan is a good fan with something like 1.4mm H2O rating and it is slowed donw it will still have something like 1.0mm H2O rating and eaisly overcome the 0.4 resistance of cooler .. meaning going push/pull makes little difference.
While Hyper 212 were decent coolers for their low price in their day, there are now many coolers just as good or better for same or lower prices .. and most of these have much better mounting systems.
Now old is the cooler and when was the last time it was thourally cleaned?