Hm.. Wondering whether you mean what you say.. The terminology:
- Airlayering: A technique to grow roots on a branch or stem, by creating a wond in the bark at the location where you wantt he roots, subsequently covering with substrate and keeping moist. Once enough roots have formed, the branch is separated from the motherplant and treated as a new plant
- Grafting: A method to combine the best of two worlds. Typically a plant is grown for the strong root system (Typically a wild variety). On top of this a plant with 'pretty' characteristics (Nice flowers, leaves, bark, fruits etc) is grown on top og this rootstock. Achieved by cutting the back of borth specimens, and sliding the sections on top of eachother. The final plant has a bottom-part of one, and a canopy of another species or variety.
- Hybrid speciation. Two varieties of the same species are crossed by fertilizing one variety with pollen of the other variety. The result is a micture of characteristics between the two varieties. The endresult is very unpredictable. This is *very* common in japanese maples, and has as a result all the different varieties we know. By carefully selecting individuals with the desired traits (Think serrates leaves, Bright fall colors or pretty bark) and further hybridizing individuals from the first cross-breed one can grow plants with traits previously unkonwn. The results of these crossing excersizes are typically not 'stable' when breeding through sexual reproduction, and can only be multiplied by grafting or sometimes cuttings (In japanese maples more often than not, the desired plants have roots that are too weak to support a tree for long, which is why they are grafted on wild rootstock).
Growing different varieties of the same species on a plant can be done (Especially in fruiting trees grown for public use one sees this, think of apple/pear combination trees). However, in bonsai we try to grow plants that resemble natural trees. Here a mixture of two types of foliage more often than not is disturbing. Also differences in bark and growing speed will through the illusion off-balance.