The sign "goodnight" is
a compound of the signs GOOD and NIGHT. When you compound (join together) two
signs you tend to reduce the movement and contacts of the original signs.
"Goodnight" is not two signs strung together, but rather it is a single sign
that is specifically used to wish someone well as you separate for the
(advanced version: compound sign)
(basic version: non-compounded)
Now, if you wished to tell someone that you had a really "good night" you could
use the separate full signs "GOOD" and "NIGHT"