Under the LineageOS 15.1 downloads folder there's stock LOS second-screen enabled builds, but I'd recommend the ResurrectionRemix second-screen builds personally, as they're much more up-to-date than the latest LOS one is.
You'll have to search for it in the ResurrectionRemix thread, but it's not hard to find.
You'll need my second-screen v71 kernel download for the H990 at the bottom of the page.
Flash the ROM and then the kernel, followed by any system modifications like Magisk/Xposed/GAPPS/Etc. Wait for the first-time boot for a bit and then you're good to go
Every now and then I update the second-screen builds with changes that Omar makes, but currently he's not too far ahead of v71.