When a fan moves air from one side to the other, this will reduce the pressure on the upstream side and increase the pressure on the downstream side. How much the pressure is affected will depend upon whether the fan is operating in totally free air, or has to move air through ducts, around obstacles, etc. A fan which can move 100cfm with a certain pressure differential across it will generally be able to move more air when the pressure differential is smaller, and less air when the pressure differential is larger, though this effect will be much more significant with some fans than with others.
If one places two fans on either side of a heat sink such that one fan pushes air into the heat sink and the other draws air out, both fans will move the same amount of air, and that amount of air will equal the amount of air moving through the heat sink. The upstream fan will increase the pressure of the air feeding into the downstream fan (thus reducing the pressure differential across it); the downstream fan will reduce the pressure of the air on the downstream side of the upstream fan (thus reducing the pressure differential across it). The fact that each fan will have a lower pressure differential across it than would be the case in the absence of the other will enable the fans to move more air together than they would individually, though how much more would depend upon the design of the fans and the ducting or obstacles between them.
If one places two fans so that both try to draw air out of a heat sink, creating a partial vacuum, and lets air flow in naturally to fill that vacuum, the amount of air that flows will be such that the total air flow through the two fans equals the total air flowing into the heat sink from outside. Drawing a deeper vacuum will cause air to flow in faster, but will also increase the pressure differential seen by the fans. Having two fans would draw a deeper vacuum than having one, but each fan would move less air than it would if operating individually.
Most likely, having two fans would increase the total volume of airflow significantly, but would not double it. On the other hand, without knowing the exact conditions under which fans are rated it may be hard to judge whether two smaller fans will move more or less air than a single larger fan (whose CFM rating may have been measured differently).