Sure they are the same model, if so, swap them round and see what happens, could be a feature of your bios.
If the speeds stay the same with rear now at 1500rpm, keep them that way, you "should" create negative air pressure by sucking more air out that you intake, for best airflow.
Yup, it was a BIOS feature. I had smart fan enabled and it was setup to power the 2 fans at different RPMs. If i disable smart fan, both fans will run at max RPM.