Even I thought at first, that dual cameras wont make a difference. Well, to my utter surprise, until and unless I started using one, I dint understand, but eventually got my ans.
First of all, dual cameras on the budgets are basically way good and can be better than your expectations these days (I have best used the KXD T55, which performs amazing and according to me, this is the best dual camera phone you can get at the lowest 10K price and still get DSLR effect photos,but still you have to know how to use it, which aperture effect you have to use to get the best result, since its a tricky).
Secondly answering you questions,, of course none of the two is better until they have better camera sensors,irrespective of the MEGAPIXELS. Clear it out, Megapixels just determine the size of your photo in pixel range, means how big it will be in width*height,, like for typical 12MP ones, the whole resolution is sometimes used up (like in Nokia N8, you can get 4000*3000 pixel photos with its 12MP). The detailing and the overall clarity, colours reproduction, dynamic range, white distribution all depends on the software and the hardware of the camera(also on the chipset’s power to handle everything with fluidity). Hardware of the camera here mostly concerns the Lens size and the sensor size. Lens size determines how much light can enter into the sensor, wider lenses have better performance in low light (if the phone’s software is capable enough), even medium size lenses perform best during macro photography (best ones are f/1.9 and f/2.0) but even now bigger f/1.7 and more wide lenses perform equally good enough during macros.
Sensor size determines how much light can be absorbed that reflects back to it from the scene/subject. Now traditional 1/3″ sensors are good enough, even now made better with better software algorithms and even used in flagships (LG G6, NOKIA 8). Bigger 1/2.6″, 1/2.5″, 1/2.3″ sensors are very good in detecting the scene and can produce incredible detailed photos, in any lighting. For Macros,the subject can be made more and more detailed like DSLRs do with bigger sensors.
Now, single cameras can perform better if they are equipped with better camera sensor and lens system (with other added accessories like colour spectrum sensor for accurate colours) with capable software. Coming to Dual cameras, here also the same. If the rear one is a good one, with the extra one being capable of its purpose (that either be a optically zoomed one or monochrome or a Depth sense) then photos can come up equally good. Typically budget phones introduce this dual cam setup for better photos with a bit inferior hardware sometimes, which kind of produce good photos but not upto their hardware. In contrast to this, some single camera phones perform better, better than the dual rear ones. And in this aspect, MP size doesn’t matter, what matters is their sensor size and software algorithms.