This image is the snapshot of the video signing "happy" in American Sign Language.
ASL sign for HAPPY
Definition: Feeling or showing pleasure or contentment; pleased; glad.
Pronunciation/sign description: Dominant horizontal flat hand (handshape), palm in (orientation), on the chest (location), brushes upward twice (movement).
Question: "When does a signer articulates with two hands in HAPPY?"
Answer: One-handed "happy" in ASL is more commonly used in general. The two-handed version is not as much used as the one-handed version in term of casual/informal registers. However, the two-handed version, when used, may have other meanings or contexts. When using emphasis, either version may be used. The two-handed version can be used in the context of singing "happy birthday" which shows more volume, but it's not always the case.
Aside from that, the variants of movement, speed, and intonation, including facial expression, may convey subtle variations of meaning, used by Deaf native and fluent signers.
Baby signing "happy"
This time-lapse video shows how the baby first learned the ASL word "happy" which emerged during the one-word stage of language development.
As the handshape is unmarked, it's easy to form the handshape but the movement may be harder to form at this stage. The upward movement in adults signing "happy" is subtle, too.