Dual-camera smartphones are relatively new in the market, but I was wondering if a camera app could explicitly choose to use only one lens, or to manually retrieve separate input from each lens.
I couldn't find any Android API documentation that is specifically designed for dual-lens phones, so I guess this is a hardware/OS-level implementation that would be difficult to override or bypass. Android's Camera HAL documentation page doesn't mention dual-lens devices as well, but it does seem to strengthen that assumption. I don't have much experience with iOS, but I guess it wouldn't be easier.
So the question is - how, if at all, would such task be possible on either Android or iOS?
Edit: seems that in iOS it is possible, as explained here (thanks to the4kman for pointing this out in the comments). So I guess the question remains for Android only.
Different vendors provide dual cameras for their Android devices in hope to improve the photo quality for average user, more often than not, specifically tuned for special conditions like challenging illumination or distortions of selfie mode. Each vendor uses proprietary technologies to handle dual cameras, and they are not interested to disclose the implementation details. The only public interface they support is a virtual single camera which is more or less compliant with Google specs.
This does not mean that you cannot be lucky chance be able to unlock the camera for some specific device. Playing with photo modes, or scenes, or other parameters, may accidentally give you a picture that only passed one lens of the dual setup.
In some rare cases some documentation is actually present. E.g. HTC let you control stereo vs. mono setting for the circa'2011 Evo 3D device.
Another example: this review implies that for Huawei P9 and P10, you will actually get one-camera result if you choose monochrome mode.
A recent article gives some perspective how different are approaches to dual camera among different manufacturers.