I can’t agree more with you, I can say that Martin already have solved my problem and doubt about it, now that I know what was my error is just a matter of time optimize all in the scene 
EDIT: obiviously thanks to all of you for the help guys, I have learned a lot here!
I am glad it’s useful. Please don’t hesitate to ask anything though. If I happen to know more on some subject I will be very happy to share my knowledge.
Yes, I am not sure how it works exactly, but the Material shading mode should be loading the textures to vRAM, so if it’s in use at the time of rendering that might be a problem.
You will have a lot of benefit! You will be able to work while rendering practically without any noticeable lag on your system for one thing. Plus it will free some memory on the GTX970.
What’s wrong with the other card? You should just plug it in if there is space for it and plug your monitor to it. There is no need to worry about 16 PCI-E lanes for GTX970 if they drop to something like 8 or even 4 if more PCI-e slots are used, that speed is only important when loading the scene, not for rendering. If there is a free slot for it and physical space, I think you will be very happy with it soon 
So if I’ve understand the old 750ti will be connected and use only and exclusively for the monitor, right? 
I don’t use that 750ti from over a year but should be ok, whats worry me most is the physical space in my chassis and if the power supply can erogate enough even for another GPU, I must check
Thanks once again for the help!
Yes, that’s right. The card used for work does not have to be good. It’s nicer to have a more powerful one, but it is really not essential - basically anything would be OK, but it makes a lot of sense to work with one and to render with a separate one. It makes a huge difference. For example while rendering in the viewport, the interface remains fully responsive since it is drawn by the card that is not busy, you also can work on modeling while other stuff is rendering,and it saves memory for the rendering card, I even sometimes work on materials using CPU rendering for preview while rendering other stuff on a dedicated GTX970. Even if you don’t have enough space there, it’s most definitely worth it to see if you can work using whatever other integrated video card your PC has.
I use Quadro600 connected to the monitor for work and GTX 970 for rendering. Quadro 600 is definitely nothing special, but it’s more than enough for work. Even if it lagged(I haven’t had such a case yet), in a terribly huge scene, there is no benefit in viewing all the scene at once while working. I switch to local view hiding everything else except the parts I work on to make it less distracting long before I would need to do that for performance reasons anyway.
Sound cool, so I’ll not obligated to work on two separates pc for different scenes during renders.
You have saved my day Martin, thank you very much!
I posted something you guys might have an answer for…
Thanks in advance