At the moment the default settings for NVS and FCB are indeed overlapping; you cannot use them at the same time. To use NVS and FCB together my advice would be to look at the board dts file and to shrink the image partitions so that there is more space at the end of flash for storage. NVS needs a minimum of 2 sectors of each one flash page. Shrinking the image partitions will probably require you to rebuild and flash mcuboot.
I would not just set the NVS offset to FLASH_AREA_OFFSET - 4096. If on your system the flash page size is 4096 bytes, the second sector of NVS will still overlap. As soon as you reach the end of the first sector you will overwrite the first flash page of FCB.
