When we apply oils as recommended by the French model of aromatherapy, we wait two minutes between oils.
The elapsed time gives each one the opportunity to move through the body, offering its array of molecules and balancing whatever it is able to balance. This means that each remains intact with its full range of chemical components – and their benefits – to offer the body. 
People often ask me whether single oils or blends are more powerful. The answer may be that it depends upon what your body needs.
Applying the oils neat (undiluted) is an extremely simple and safe way to utilise pure, premium essential oils. One does not need to understand the chemistry in order to safely derive the benefits. However, the oils must be genuinely pure, premium oils that contain no solvents or other additives.
As we apply multiple oils, our bodies select exactly which components are needed from each one to come to balance. The body then eliminates the remaining components through the urine, feaces, saliva, and perspiration. Simple? Yes!!!
When a blend is created, the individual essential oils chemically interact with one another before they reach the body. In other words they create a “new substance” which may have properties that are quite different from the properties of the individual oils. Some of the individual components may be neutralized in this process. Therefore, one cannot assume that the blend will have the properties that they may desire just because it contains some of the single oils, which have been shown to have the desired effects.
If the person creating the blend is knowledgeable about the chemistry, then the essential oils may combine synergistically to create something which is exceptionally powerful – perhaps even more powerful than applying the individual oils. 
This is illustrated by the action of the blend, R.C, which was created by D. Gary Young. Not one of the single oils in the blend (eucalyptus globulus, eucalyptus radiata, eucalyptus citriodora, myrtle, pine, spruce, marjoram, lavender, cypress, and peppermint) has been shown to be as effective as the blend, R.C.
I always teach that LAVENDER is the greatest substance known for healing skin. Lavender is in RC. Does that mean that RC would be helpful? Probably not.
I would always stick to the single oil of lavender for healing the skin.
In summation, be sure you understand the effect of the combination of oils in a blend if you plan to use blends. The Essential Oils Desk Reference is an excellent source of information for the blends made by Young Living Essential Oils. In addition, many have shared their experiences using blends for various conditions, and they are often included in “Tidbits”.
SO . . . which is better – single oils or blends??? It all depends upon what YOUR body needs at any given time. Remember, essential oils work with the body to bring about harmony and balance.
Frances Fuller