drivers – Can not I use several graphics cards at the same time? [Ubuntu 18.04.1 LTS]

So I'll try to make this as short as possible, but I have to explain a little. I recently updated my computer by transferring it to a new and better case that gave me many features that my previous case did not have. Most of it was for the updates I wanted to do in the future, but one of the things that I was allowed to do for now was to fit in a second GPU. I dug up my old Nvidia GTX 750 Ti and launched it to the system, and I can work together with my current AMD R9 270. This also allowed me to connect my second monitor to the GTX card. I could go on and on about how this allowed me a lot more freedom while playing videogames, since before this I could not use my second monitor at all while playing videogames, since my frame rate was reduced a bit by doing basic tasks I like to see a YouTube video, but now I can make my GTX card do that so I do not put any additional pressure on my R9. This also allowed me to use OBS Studio with the NVENC codec, which has made my recordings of higher quality but smaller in file size. This is also good because Shadowplay and ReLive are nightmares to deal with, however, they were the only programs that would allow me to record without my FPS being cut in, like, 90%. Before I could use OBS with my second card, I had to take a long break from Linux since Shadowplay and ReLive are only compatible with Windows, so I'm sorry if I do not know what I'm talking about with Linux.

With that out of the way, let me start talking about my problems. I used this configuration for a while on Windows, and both my monitors and OBS worked fine. After I confirmed that it was working, I went and installed Ubuntu (18.04.1 LTS) in a dual boot with Windows. However, when I went to use Ubuntu, my second monitor did not work. Confused, I went to install the proprietary Nvidia drivers, not only with the hope of solving this problem, but also so that OBS could use the NVENC codec. Even after restarting with the new installed drivers, it still did not work. However, OBS worked well with the NVENC codec. I thought it would make sense if my second monitor did not work, since I'm using it on a separate GPU made by a company completely different to the GPU of my main monitor, so I decided to try it on my integrated graphics since my CPU is an AMD CPU and , of course, the iGPU is AMD. But even after a reboot, the monitor STILL would not work on the iGPU. I know that there is no way it is a problem with the configuration of my hardware, since it works perfectly well in Windows, obviously both cards are connected and work, and there is not even an option to disable the iGPU in my BIOS. Given all this, it's obviously a problem with Ubuntu. I know you are using the card since OBS can use it for encoding, but you are not using my second monitor. I have checked the screen settings in all my attempts, but I would not even recognize that there is a second monitor connected, and I would try the Nvidia X Server Configuration to solve this problem, but the application is not even starting. Someone who has some idea of ​​what is happening and how to fix it?

Thank you,