First of all: those objectives are fixed-focal.
One objective being 56mm means that it does not see anything around the frame what 28mm objective sees. You cannot interpolate between something known and something unknown, it is not interpolation.
There is only one convenient way in which this can be implemented:
- just one objective is used to record images at 28mm (or may be slightly past 28mm) or 56mm (those are 135-equivalents BTW)
- each image in-between has interpolated 28mm frame data (one may say "digitally zoomed") with 56mm frame data mixed into the central part to improve resolution of central part of the image in such a way that a resolution increase is not apparent at transition from wide objective data to tele objective data
- the resolution of output image being the same even though it does not mean anything at all: the outer part is interpolated to match one resolution throughout the range and the inner part is just partially mixed in to improve details
It is same interpolation which is used for digital zooming but it improves the central part of the image using "tele" objective.
Regarding the distortion: of course Apple would make it seamless too, it is a part of the trick. I hope that you do not mistake perspective as distortion, those are different concepts: perspective change results from shooting point change, geometric distortion is a property of optics. Perspective is exactly the same with any objective given that the shooting point is the same while distortion depends on the optics.
Usually the cropped central part of image made with some objective does not feature more distortions than image made with longer objective - the most distorted part of images is left outside of cropped area. I.e. second objective most probably does not improve distortion itself, it just improves detalisation.
Also geometry corrections are implemented in wide range of cameras for years already, so this means there are no real reasons for 28mm to exhibit more distortion at all. I do not know whether iPhone corrects distortion though.