Hello,
My understanding is that only one works at a time. In other words, if you left FE set at 6 but switched to F2 only F2 will be engaged and it doesn't really matter what FE was on - zero or a 6 and vice-versa with F2 switching to FE.
I have read that a setting of F2-4 is the equivalent of FE-0. So, if you want minimal masking of non-ferrous items in an "iron patch" you would use F2-0. You may want to try differing recovery speeds in combination with a setting of F2-0 or a combination of lower F2 settings and recovery speeds until you find what works best for you.
I think I would try f2-2 and recovery speed of 6 to start, go slow and see how that works.