Fans can be back-pressure sources (this is the case of squirrel-cage blowers, like used in vacuum cleaners), or they can be volume-of-flow sources, or anything
inbetween. Probably the familiar radial-blade air mover is most near a
volume source (unless airflow is constricted somehow), so a second similar
fan in series might rotate like a pinwheel, neither adding to the air velocity
nor retarding the airflow (and not taking much power).
The tuyeres of a forge may act as a significant backpressure, and the classic
bellows that a forge uses, is a fixed volume source with nearly unlimited forward pressure (because the apprentice works hard!). A vacuum-cleaner style air source could be run with a speed control to drive tuyeres. A single PC cooling fan might not push hard enough, but several in series will to some extent share the backpressure load. Adding fans in series won't add to the volume per minute of the single fan in free air, but might counter backpressure.