The easiest thing to do is to wire the two fans in series instead of parallel. Unfortunately, this will also cause the fans to go slower, so they might not be able to cool the hood anymore.
Series:
hot -----------FAN------FAN----
|
nuetral---------------------------
Parallel:
hot -----------------------
| |
FAN FAN
| |
nuetral------------------
OK, so the ASCII art isn't great, hopefully you get the idea.
If you need all of the air flow you currently have in the hood, then you will have to replace the current fans. There are differences in the amount of noise that different fans make, and most fans list the amount of noise that they make as one of their specs. Unfortunately, quiter fans are likely to move less air. So you might have to go to a bigger fan to get the same air flow with less noise.
