[Galaxy S 4 i9505 Original Android Development] royredman:
ok, I don't know if your phone is supported (and you need to check that in the download section of supported devices first). I'm going to assume your phone is supported and will explain it to you (but check first).

Anyways.. yes, you can use your stock rom as primary. You don't need to do very much with your primary rom to prepare it for dualboooting. You need to install the dualboot app (in the downloads section) before you can dualboot. Once you do that, you want to open the app, and click on the rom icon (when you tap the side bar on the right). When you are in this section, it will most likely ask you to set your kernel. Say yes, and that is pretty much all you need to do with your primary rom to have it set up.

yes, you can use pretty much any other rom as dualboot (including stock, aosp, CM, etc.). When wanting to installing the dual rom, you need to patch all of those files first. On the sidebar, tap, "patch zip file". Choose your device (make sure its the right type of s5 (LTE vs Tmobile vs AT&T)), then select if you ar installing a second rom spot, or third, etc. Make sure to patch EVERYTHING you plan to flash onto your second rom. This includes Gapps, separate custom kernels, and the rom itself. after that is all done, its now the fun part, flashing the rom

You go back to the rom tab (where you setup your primary kernel). Then tap the red icon on the bottom right corner. After that, click the red plus sign in the same spot, it will ask you which files you want to install, then when you click it it will ask you if you want to change the install location (from dual to multiboot 1 or multiboot 2, etc.), odds are you have the ideal set location since we just patched it, so, say "keep same location". Add any more zip files you need to (Gapps, kernels), then click the checkmark icon on the top left when you are done. It will then flash the rom, and if it flashes fine, it will say it was flashed successfully.

now, in your rom section, it will have a checkmark on the second rom now instead of primary (this means that your next reboot will load you into that second rom instead of the primary as default). You can easily change that by tapping on which rom you need to reboot into the next time you reboot. Be careful though, because the first time you install a another rom slot, you need to reboot into it to set the actual (second) rom up.

So, now you need to reboot into your second rom and set it up as you would normally do with a new flashed rom (setup your google account and apps). Once you do all that stuff, install the dualboot app on the second rom (VERY IMPORTANT). Open the app in the second rom and set your kernel under the second rom icon. Once you are done with that, you are all good to go. You can switch your primary rom, by tapping on the rom icon in the Rom section of the app and then rebooting into that default rom selection.

There is another thing that is awesome called app sharing that i can help you with later if you get everything up and running properly (assuming your device is supported). If you have anymore questions, feel free to ask, but I think I gave you a really detailed explanation of this process. Oh, also, be careful about the size of the rom, because if it takes too much space in the system size, the rom will not load. If you have a problem loading the rom, use the dualboot utilities (in download section) and flash it in TWRP. You can delete the rom there and load back into your primary rom pretty easily.

Good luck and happy flashing!!