first is uboot-image (not preloader) for SD-card and must begin at 2k (skip 2048 byte) as the name says.
second one is preloader for emmc (0k) which must be written to boot0-partition starting at byte 0 (from beginning)
maybe it’s needed also for SD-card…have used my SD-Cards with jessie image and replaced only uboot, kernel and OS-files
the 0k/2k says where the image starts, see https://github.com/BPI-SINOVOIP/bpi-tools/blob/master/bpi-bootsel#L230 for details
for SD-Card-image https://github.com/BPI-SINOVOIP/BPI-R2-bsp/blob/3823964822219743e0fe785e04832813726862a1/scripts/bootloader.sh#L26 is the reference, here preloader_iotg7623Np1_emmc.bin is used