Making Time Machine scan/back up a separate NAS

I'm slowly loosing the will to live in achieving what I believe should be achievable! Would really appreciate any help or advice anyone can offer.

Like most people I'm paranoid about loosing my extensive picture and home movie library and I'm not yet comfortable with cloud solutions given the weeks and months it takes to upload it, let alone recover it. So my preferred solution is a NAS system hidden and locked in a modified fire proof box attached to a powerline network adaptor in my home. We've been using powerline adaptors for a while and the data rate is pretty good.

I have 2 x MacBook Air and the wife and I share our iPhoto and iTunes library's which are stored on a separate NAS (i use a TC to look after these) and accessed via Ethernet (they can be accessed wirelessly, but the fragility of iPhoto means we use only cabled connection when playing with pics)

In short my MBA's are nothing more than "interface" machines with all our content stored on 2 NAS and Dropbox.

Now here is my challenge, I obviously want to back up primarily my iPhoto and iTunes folders to my secure NAS, tucked away nice and cosy in its fireproof box. But for the love of sweet Jesus I can't seem to get Time Machine to see it (the NAS (TC) holding iPhoto & iTunes) in order to back it up to my fireproof NAS which is also a 3TB TC.

Everytine I go onto TM it asks me to select the back up disk, I select my fireproof TC and it begins happily backing up my MBA, which while handy, really isn't that critical to me, given I'm using it only as an interface device to use and consume content rather than store.

I'm sure I'm not doing something and I've been all over google and various forums but I just got lost in messages about using TM to back up to a NAS.

In short, I need TM to scan TC 1 (iTunes & iPhoto libraries) and back it up to TC 2 (nuclear proof back up NAS).

I'm running a July 2012 MBA with Lion 10.8.

Any help or advice would be really very much appreciated.

Many thanks