More fans == more noise.
Fans make nore noise when creating a vaccuum in the case.
Ideally air flows through a case from front to back...
If your case has adequate air intake at the front, you probably only need one medium speed (2800rpm) fan at the back, right behind the CPU.
Also, you may find that covering unused rear exhaust holes will improve overall cooling. What happens is that the exhaust fan and power supply fan will draw air along the path of least resistance. So, if you have an open fan position only a couple of inches from a running fan the air will enter there and go right back out the fan without circulating air inside the case very much at all.
Another common thing with cases is that, although they have front fan openings, very few have good air intakes at the front. You may wish to look into enlargening the existing front air intakes, or even cutting new ones in out of sight places, whether you use an intake fan or not. I commonly remove the plastic front cover and cut away the underside edge, creating an opening underneath the case (where it's out of sight) that substantially increases airflow into the case.
No harm in using 2 fans, so long as the front air inlets are able to let in enough air to ensure both exhaust fans are working at full airflow.
Looking at the mounting brackets for the fans, I get the impression you might run into vibration problems, which will make lots of noise. You may want to consider a way of preventing that... perhaps by removing the plastic clip in thingies and mounting the fan(s) directly to the case.