Problem with multiple NICs and IP Passthrough

I need IP Passthrough for my server but I also need to access it from the local network… So I bought an extra network card and thought I could connect that to the internal network and the main NIC straight to the modem for that IP Passthrough. Unfortunately, when I configure and connect the second NIC the main NIC shuts completely off after a while.
Only removing second NIC from /etc/network/interfaces and restarting networking service helps to bring the main NIC back up.
What am I doing wrong here?

Server is running Debian testing
(Linux strangelove 4.7.0-1-amd64 #1 SMP Debian 4.7.8-1 (2016-10-19) x86_64 GNU/Linux)

Someone said I should update to testing which I did, problem still occuring.
That someone also said its the drivers. I’m saying its more of a PEBKAC or something in the configs.

NICs are (according to lspci)
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 06)
03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 02)
Not sure which is which…
The motherboard is a Gigabyte GA-F2A85X-UP4

EDIT: managed to make it work

What was the Problem ?

Some black magic thing.. I can't remember what I did to it but it started working eventually..
Probably the combinaiton of some config somewhere pointing to another config that was set wrong and so on and my modem being conffed wrong as well.

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