thanks for raising and digging into this issue!
I've tried to reproduce your steps using Virtualbox with grml64-small testing
build1641 (see /etc/grml_version). It indeed works when using only *one* network
interface, but as soon as using two network interfaces /etc/resolv.conf is empty
for me, so that seems to be part of the problem.
FTR: as soon as we run Grml with systemd (instead of file-rc) this issue is
gone, though the migration to systemd isn't finished yet.