Dakar,I think you need to put it in lamens terms for Bobby,LOL.Did you combine each wire from the 2 fans together then atttach it to the converters wires?.Did you check the mA rating on your converter and see if its putting out enough juice for each fan?
Dont forget to double the mA rating for the fan since your installing 2.Your fans are probably around 150mA so get a 300mA converter @12vDC.
And lose the LED's on your fans,they are probably whats giving you trouble.