In my headcanon of a Sparkling. When two Mechs bonded, their Sparks did connect so much, but that's if two of them opened their spark chamber and connected to each other.
Whoever the carrier is always the strongest one and the other as Sires. The sparkling are created from little parts of the Carrier's body parts to form a Spark hub.
They have 5 stages of process:
Phase 1: spark hub latched to Carrier's spark chamber, suckling the energy from the carrier's spark.
Phase 1.5 - the sparkhub would be super too sucky sucky with carrier's spark and would make the Carrier's energy depleting a lot. That's when the Sire becomes a temporary Spark carrier till Carrier regain energy.
Phase 2: Spark hub grew bigger, transformed a little into a tiny protoform (imagine tiny bug). This is where they are the size of a palm this form made them still unlovable and still latching to the spark chamber. Bug sparks start to nibble Carrier's spark wiring and again still in suckling mode.
Phase 3 : This phase is a little critical in need both Sire's help too as they gotten more aggressive at eating their spark out. Sire have to provide a hub out of their chest ( the bigger the chest, the better to provide space for them,) the smaller sires would make a temporary hub so they can attach their spark wires with the little bug protoforms. Imagine they carries like how Opossum carries their children or... Spiders.
Phase 4: Sparkling is evolving. They had started to transform their bug body into a pair of tiny Legs and arms. This phase needed the two sides to again put them in their case. Carrying them till they opened their eyes. The Carrier can still carries them depending on how willing they are but at time Sires can reverse role with the Carrier.
Phase 5: Protoform ready to pop out. They are now eyes opened and ready to leave the nest. The Sparklings would start walking instantly.
CONGRATULATIONS! YOU FINISH YOUR SPARKLING INITITIATIVE PROGRAM!
Below: my sparkling AU featuring Elita One and Optimus Prime.
In this AU, Elita-1 and Optimus Prime are in a relationships. Then... Oopsie happened and they are now Carrying.
Hearing the news, Of course, Bee, is hundred percent happy they got a sparkling together.
No they do not know what's an Uncle. Haha
Elita-1 in the Phase 1.5-2, Sparkhub starting to get very aggressive with the spark sucking.