This is related to: What is the difference when a jutsu is performed by weaving only some of it's seals and when performed by weaving all of them?
According to the answer, head-weaving is a technique used to manipulate chakra to perform the Jutsu. Some users require more hand seals to perform the Jutsu whereas other do not require as many due to their chakra control.
In Naruto Shippuden, Episode 374
We see Kakashi and Obito having their battle in the realm where their Mangekyou Sharingan takes them. At roughly 8:30, Kakashi and Obito are in close quarters and Obito takes Kakashi's hand to perform the hand-seals for FireBall Justu.
How does the charka manipulation through hand-seals work when performing them with another participant?