I'm French and my dad is a huge fan of good restaurants. This means that, when we go on holiday, we usually eat at a good restaurant at least once.
For my dad, the definition of a good restaurant is a restaurant that serves traditional French food (or German when we are in Germany, etc..) and he usually chooses them by looking in the "Gault&Millau" (a list of good restaurants).
So far, we didn't go to the more expensive ("gastronomique") restaurant so I don't have experience with the really expensive restaurants (just some really nice and a bit expensive ones).
Here is what I have observed
Sharing dessert is always okay. Whether you choose to take "one for two" or to do a "half-half" (you take two then each person eat half of each), you won't have any problem or be badly seen. Also, the sharing technic here really doesn't matter as nobody cares.
For the main course, having someone else taste a little of what you are having is also a very regular thing to do. Also, I'm often unable to finish my plate in the restaurant and my family always finish it for me which has never been a problem (as far as I can tell, it's also a regular thing to do).
About the fact that you decided to eat half and then switch plate, I must say, I find it kind of unusual. But is it bad etiquette? I really don't think it is. After all, when I want my sister to have a taste of what I'm having, we often switch plate and then switch back when each of us has taste the other food.
About the idea of transferring half of the food...
...from one plat to the other
Please, avoid doing that. You will completely mess up the presentation (when you are transferring food to another plate, since you are transferring a lot of food at one time, it's really difficult to be "delicate" with the presentation in order to not ruin it).
Taking a bit from someone else plate is fine. Switching plate is fine. Telling the staff in advance that you will each have half of what the other is having is an option (even though I have only seen this happen for dessert).
But messing up some plate presentation?
Don't do it, you will likely be seen as rude and uneducated if you do (People always look at me weirdly and kind of disgust when I dare mix my "floating island" dessert).
Add per requested about the "not mixing rule":
This is probably a cultural thing but I was taught to not "mix" my food (unless it's already is or is meant to be mixed). You can eat in the order you want when things are in your plate but people will definitively see you as weird if you mix some stuff that isn't supposed to be mixed (at least, that what I have observed). And, even when something is supposed to be mixed, I will often only mix a small part at a time to respect the presentation because I know it's important to the people around me.