The rule is pretty simple:
When an adjective (or participle) relates to a preceding or following substantive, it must be written in lower case (it's not considered a substantivized adjective then). Note "relates" doesn't necessarily mean "in the same sentence" here - it means "context".
(Regeln des Deutschen Rechtschreibrats §58/1)
In your example, it's clear that both "gute" und "schlechte" relate to "Nachrichten".
This is even true for relations crossing sentence borders:
Der Verkäufer zeigte mir seine Auswahl an Krawatten. Die gestreiften und gepunkteten gefielen mir am besten.
Here, "gestreift" and "gepunktet" obviously relate to "Krawatten", so are not considered substantivized adjectives.
This is opposed to impersonated adjectives that don't relate to a substantive in context
Am Totensonntag gedenken wir der Toten
(There's an no substantive in context that "tot" could relate to here - Thus it's considered a substantivized adjective, thus upper-cased)