If you try to configure more than one network bond in Debian 9 Stretch you will get errors when trying to bring up the second bond – this is due to the fact that only one bond is configured by default. In order to support more than one bond you need to make the following change:
Edit /etc/modprobe.d/bonding.conf and add the following:
options bonding max_bonds=2
… set the number to number of bonds you require.