'Lo guys, and thanks for the tag @Lingogrey
The "tongue map" is indeed a myth, and quite a ridiculous one. Place a pinch of salt on the very tip of your tongue, and you'll immediately taste salt. Primary school teachers try to rationalize this by stating that your saliva can instantaneously transport the molecules to the correct area of the tongue... which is just a tad ridiculous. Here is a decent article explaining some of it:
http://www.livescience.com/7113-tongue-map-tasteless-myth-debunked.html
And yes, as @Mike said, perfect pitch, 20/20 vision, super-taste etc.. is not an indication of better or worse, although we often confuse these as such. For instance, my wife (a professional musician) has perfect pitch, and it drives her crazy when an instrument is tuned to 455 Hz instead of 442 Hz (the natural and open "A"). However, her lowest audible sounds which she can hear is surprisingly loud, which would be classified as bad hearing in any other case. In the same vein, I've met musicians who can pick up amazingly small variations in pitch, as long as they have a suitable reference frame (in other words, another pitch).
The same goes for taste/smell, except at a whole more complicated (in terms of chemistry) level. We've been able to manufacture digital eyes (cameras) and digital ears (microphones), but digital noses are still beyond our reach. In fact, there is a whole branch of analytical chemistry dealing with olfactory identification of compounds in conjunction with traditional analytical chemistry techniques, due to the fact that the nose can detect ridiculously low concentrations. Unfortunately, the nose/mouth is also subject to large number of chemical phenomena, which increases in complexity when we start dealing with real world examples. The taste and smell receptors are also subject to a wide range of individual, environmental and racial differentiation, which makes exact comparisons between taste/smell evaluation between individuals as well as between compounds very difficult. The best which can usually be done in these situations is to train your own senses to exact reference frames - which is how wine masters and super-tasters are developed.
The reason I diverge into this is to yet again, highlight the massive differences which individuals can experience when tasting wine, food or e-liquid. And this does apply to 'taste-regions' as well. I definitely have a different experience when I'm vaping directly onto my throat, tongue-tip or tongue side. However, I don't necessarily taste any particular note differently - it is more in terms of different mouth-feel. Similar to wines, where high tannin content tickles the side of your tongue, or depending on the composition, the back of your throat (eg. Shiraz have a very typical tannin profile on the sides of your tongue, which you'll get less often with a Merlot. It is one of the easy cheats in cultivar identification!It might even be a factor of the nicotine or additives rather than any flavour concentrates.
Anyway, this has been slightly off-topic, so going back to the OP:
Getting different notes in a juice during inhale or exhale is a bit more difficult than one might think. Certain concentrates/compounds already have these attributes - for instance, most of the creams I find developing more on the exhale, where slightly acidic fruits I get more on the inhale. Unfortunately, to change the nature of a specific flavour (in terms of exhale/inhale) will be very difficult to achieve through steeping/flavour composition alone.
The entire point of steeping is to
a) break bonds between identical flavour molecules, which tends to aggregate somewhat
b) form new bonds between different flavour molecules, thereby suppressing specific flavours whilst developing novel ones
c) form new bonds between flavour molecules and solvent (PG/VG/Nic) molecules
d) allow any necessary chemical process (such as redox reactions or any intramolecular side reactions, usually through UV or heat) to occur, which can still alter the final flavour.
Now, all of these processes (which can tally exponentially, depending on the matrix of compounds in the mix) are subject to specific equilibria. Which means, in principle, that when you add a specific concentrate at a later stage, the entire system (including already developed flavour notes) will trend towards a new equilibrium, and you'll have a final solution more or less the same as if you would've mixed everything together right from the start.
The exception would be if you have equilibria in the solution which are very strongly favoured towards one side of the reaction. In this case, you might be able to trap a specific compound in a specific state if you have limited quantities in the initial solution. To give a practical example using a few known flavours (but really, this is not how these flavours work - I just want to illustrate):
Lets say strawberry concentrates can react with creamy concentrates to form a new flavour profile which is more creamy than strawberry - in other words, less tangy. But on the other hand, strawberry can react with other fruity (eg, dragonfruit) concentrates to create a more tarty, tangy taste. If one of these reactions tend to form mostly irreversibly ( in other words, every drop of strawberry concentrate will react with every drop of cream to produce the new and mostly robust flavour), then a staged steeping setup would be possible. Using the same type of principle, if you would be able to 'dope' menthol with a specific compound so that it only releases at high-heat, or it only reacts with saliva, or oxygen, or whatever - then yes, what you want would be possible.
Of course, the type of flavour concentrates used are a) generally undocumented and ill-researched, and b) quite complex - more than one type of molecule in a single concentrate. Therefore, to understand the chemistry of these concentrates and develop a scheme on how to achieve what you want, would be at least a few years' of research, and then only without taking the biology of taste and smell into account.
However, mixing e-liquid - like cooking - is rooted in chemistry, but it cannot be fully explored at that level. In other words, a certain amount of art is intermixed with experiment. So despite the practical impossibility of trying to figure out how to do what you want to do in a purely scientific manner, it might be possible to achieve something along the lines of what you want - as long as you are willing to do many, many, many experiments, have a good sense of taste/smell, and be willing to give up if you are not lucky enough to stumble on the winning combination. However, I wouldn't bother with staged steeping - I don't think you'll find any luck there.
