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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 one sign with two hands in HAPPY?"
Answer: One-handed "happy" in ASL is commonly used. The two-handed version is not as much used as the one-handed version in term of a casual register. However, then two-handed version, when used, has other meanings or contexts than the casual usage. It can be used as an emphasis. "Well, but she's happy." Another thing is that the two-handed version can be used in the context of singing "happy birthday!" which shows more volume.
As for the emphasis or stress, either one-handed or two-handed versions may be used. The difference in stress is the variants of movement and speed.
Review 101 words. Refresher.