O Network Manager ignora as NICs mencionadas em /etc/network/interfaces
por padrão. Portanto, não adicione nada sobre as outras NICs nesse arquivo. De man NetworkManager.conf
:
[ifupdown]
This section contains ifupdown-specific options and thus only has effect
when using ifupdown plugin.
managed=false | true
Controls whether interfaces listed in the 'interfaces' file are
managed by NetworkManager. If set to true, then interfaces listed in
/etc/network/interfaces are managed by NetworkManager. If set to
false, then any interface listed in /etc/network/interfaces will be
ignored by NetworkManager. Remember that NetworkManager controls the
default route, so because the interface is ignored, NetworkManager may
assign the default route to some other interface. When the option is
missing, false value is taken as default.