[ubuntu] Route certain traffic to second NIC with unclear gateway

There are two Ubuntu Desktop 15.10 machines. Both run on virtual machines based on VmWare Fusion.
Each of these two virt. machines uses a network card of Fusion's NAT type.
The network traffic generated by regular work goes to LAN and Inteet through such card and all are happy - users and administrator.

Nowadays one change in setup is sough - it is about downloading package updates / do installation of new packages through
a proxy - apt-cacher-ng is going to play the proxy role and is going to be installed and operate on one of these two virt. machines.
Whole package management traffic (update/installation/...) should run through apt-cacher-ng, inclusively the OS' hosting apt-cacher-ng.
In further future more clients will join (some of them are also virt. machines on same physical host but
possibly housed by VirtualBox, others are physical clients emerging in same physical LAN the physical host of mentioned
2 virt. machines is present in). But now let's limit this discussion to these two virt. machines mentioned in the begining.

Problem: apt-get update on client virt. machine can't coect to apt-cacher-ng -- it fails in the DNS dialog.
Clients apt-get is configured such a way to speak to apt-cacher on another virt. machine instead of public repository

cat /etc/apt/apt.conf.d/01proxy 
Acquire::http::Proxy "http://ip6-[fe80::250:56ff:fe2e:2411]:3142";

Thus the idea came to install on each of these two virt. machines additional NIC - this time of Fusion host-only type.
Then to route traffic generated by distro's package management via these special NIC on both sides of coection.

However the routing table is to be filled out using gateway ip. The problem is Ubuntu does not report any
gateway for second NIC, so it is unclear which ip is to be put as gateway into routing table for special routes.
Any ideas?
apt-cacher-ng is listening on port using ipv6.

Here few details on apt-cacher client machine

cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

 netstat -
Keel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         172.16.43.2     0.0.0.0         UG        0 0          0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0
172.16.43.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0
172.16.170.0    0.0.0.0         255.255.255.0   U         0 0          0 eth1

netstat - --inet6
Keel IPv6 routing table
Destination                    Next Hop                   Flag Met Ref Use If
fe80::/64                      ::                         U    256 0     0 eth0
fe80::/64                      ::                         U    256 0     0 eth1
::/0                           ::                         !n   -1  1    37 lo
::1/128                        ::                         Un   0   3    14 lo
fe80::20c:29ff:fe9b:dd3/128    ::                         Un   0   1     0 lo
fe80::250:56ff:fe3e:3cb/128    ::                         Un   0   1     0 lo
ff00::/8                       ::                         U    256 2    40 eth0
ff00::/8                       ::                         U    256 2    41 eth1
::/0                           ::                         !n   -1  1    37 lo