Supporting iPhone 6 and iPhone 6+ with different launch/splash screen image for iPad Portrait and Landscape orientations
You don´t have to use the launch screen file to make your App iPhone 6 / 6+ resolution compatible. Instead, you can select the LaunchImage asset as your Launch Images Source.
It can be found at "App Icons and Launch Images" under your Targets:
If there is no LaunchImage asset just go to your Images.xcassets, make a secondary click (right click) and select "New Launch Image":
The result is something like that:
Now just drag and drop your images for the specific resolutions you want to support and set the created LaunchImage asset as your source.
Hope it helps
Asset catalogues currently work in landscape mode on iPhone on ios8. I had this reponse from apple support:
"There is a bug involving launch images in asset catalogs and apps that launch in landscape on iPhone. Behind the scenes, the asset catalog compiler generates the same UILaunchImages key  in the final Info.plist that you would have added when you were specifying launch images manually. One of the sub-keys for each launch image specified under the UILaunchImages key is UILaunchImageOrientation which is always set to Portrait by the asset catalog compiler. This makes sense because apps on iPhone always launch in portrait orientation . However, the iOS app launcher decides that since your UISupportedInterfaceOrientations only contains UIInterfaceOrientationLandscapeLeft and UIInterfaceOrientationLandscapeRight, it will only look for landscape launch images under the UILaunchImages key, of which it finds none.
Since there is no way to force the assets catalog compiler to specify
UILaunchImageOrientationsub-key of iPhone launch images, you should continue to specify your launch images by editing the information property list for your app as before."