12V fans in series with 25 A supply
I suspect they will start working if you put a capacitor from the center point to ground. Maybe about 330 or 470uF.

Each of the fans has an electronic circuit which pulses the  coil in the motor to start a rotating field. Without a good ground for the upper fan (or without a good Vcc for the other one) this logic circuit won't work well.

It's not an ideal situation, as on Power-On, the top motor will then receive 24V for a fraction of a second. 

EDIT: @Asmyldof posted a good comment: Use two capacitors in series over the 24V supply and the center to the center of the fans. That'll keep the center..., well, centered.