Same here ! Only one works.
If i use the mac of the realtek lan, the realtek works.
If i use the mac of the intel lan, the intel work.
They work only as one, not both of them work. I have to set mac1=MAC, when i set mac2=MAC (this isn't working).
my grub:
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serial --port=0x3F8 --speed=115200
terminal_input serial console
terminal_output serial console
set default='0'
set timeout='1'
set fallback='0'
insmod part_msdos
insmod part_gpt
set locale_dir=$prefix/locale
set lang=en_US
menuentry 'Synology DS3615xs' --class os {
insmod fat
linux $img/zImage root=/dev/md0 syno_hdd_powerup_seq=0 netif_num=1 HddHotplug=0 syno_hw_version=DS3615xs vender_format_version=2 vid=0x0781 pid=0x5580 console=ttyS0,115200n8 sn=C8LWN00821 mac1=408D5CFFDEDA mac2=408D5CFFDEDG withefi elevator=your_magic_elevator quiet
set img=$prefix/../image/DS3615xs
initrd $img/ramdisk.lzma
}
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