Still don't own. The details of TF smex.
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Twins are a rare mutation. You see, when a Transformer wants a creation, they do not always need a partner to, well, create one
When a Cybertronian creates a new being, they first must visit a medic to see if their spark is capable. A capable spark must be of a certain density to form new sparks. The denser the spark, the better chance of a successful creation. If the spark is dense enough, than the next step would be to build a proto form. Any mech or femme can do this part. It is the easiest part of the entire creation process.
Next, they would need to either interface through spark contact with another Cybertronian, or, if their spark is able, they can receive a download. The latter is a very rare form of procreation. It is an extremely risky process. The only mechs I can think of right now that have had the download are Blaster and Soundwave. This is why they have so many offspring. They can access the data at any time to form a new spark.
If the mech of femme choses to interface, they can only create one spark from that interface. The spark would be large enough to be housed in a full size body, and be treated as any other full-sized Cybertronian. This process has about a 12 chance of going awry, so it is the preferred method, not to mention it is far more pleasurable to both parties involved.
The information received from either the download or interfacing acts much as a 'sperm', and every able Cybertronian has a section of their spark, if it is dense enough, that acts as a dormant 'egg'. The spark recreates this part of itself, providing a nearly limitless number of child sparks.
Once the spark is begun, it will take between fifty and seventy orns - an orn is about thirteen days, so it would be between 1 4/5 to 2 1/2 Earth years - to mature. After it is matured, the process becomes dangerous. In this war, that I know of, there was only one real set of twins. Rumble and Frenzy were planned as two separate sparks.
When a spark is separated from its creator, the creator is offline. The process is too extensive to explain fully, but basically, the spark is taken from the creator's spark chamber and put into the proto form. While out of the protection of a spark chamber, anything that touches the spark has the potential to damage it. It is a risky procedure.
Normally, the spark bonds to the new proto form and a new being is born. They are born with basic knowledge, and must be taught some things, depending on what role they are to play in society.
When a set of twins occurs, the spark that is placed in the chamber of the new creation instantly seeks to return to its creator. When this happens, part of the spark will wish to remain while part seeks to leave. The spark will divide itself, forming two half sparks with two different personalities. From what studies have shown, the spark that wishes to stay normally has the more lenient personality - Sideswipe - and the one that wishes to return to the creator will be less stable - Sunstreaker. There are always extra proto forms kept at medical facilities for this circumstance.
All this leads up to why the twins' love for each other is not incest. You see, because they were two halves of the same spark, their sparks actively seek their other. It is an accepted fact that if we lose one, there will be nothing short of a miracle that can save the other. They can barely stand to be in different rooms. The twins were created as what humans would call soul mates, only much, much deeper than that. They are one being with two minds.
There are two types of bonds; twin and mate. A twin bond works much like an eternal calling. One spark calls out and the other answers. They will be able to read each other's thoughts from their initial online, and can never hide from the other. They love one another and will do anything to protect their twin, though they have no control over who their twin is. Though they can still take another lover, that lover would have to take them both, because a spark cannot bond to half a spark.
A mate's bond is similar to a twin's bond in the sense that they call out to each other and can read one another's thoughts and feelings, and comfort each other when necessary. The main difference is that a spark mate is chosen, so their sparks will always seek to help their significant other, putting them self in harms way for minor things in the beginning, until their spark gets used to having another spark so near it. It is a weaker bond, but still just as important. A spark mate has a 55 chance of living through their mate's death, suicides included, while a twin has a 2 chance of living through their twin's death, suicide not included.
This is Cybertronian reproduction as it has always been and most likely always will be.