Conflict combining Projection and Cubemap layers using Vulkan

In my current project, based on the native Mobile SDK and Vulkan, I'm attempting to combine a regular Projection layer with a Cubemap layer. As far as I know, everything is set up correctly - from the creation of the swap chains, filling them with images, up until the creation & submitting of the layers themselves. Something is wrong, however - I can either render the Projection layer or the Cubemap layer, but not both. (Just to be clear: I *did* remember setting .LayerCount = 2 in the ovrSubmitFrameDescription2 instance, for the test. Also, I *did* remember to specify the cubemap layer first, and the projection layer second).

Since both can be rendered separately, but not together, I can only think I misconfigured the swap chains somehow, so one blocks the other in some way. Where should I begin checking things in my code to attempt to find out what's going on?

So far, I've been unable to find examples of code that performs this exact task using Vulkan - where is a good place to find such examples?