When a word can use both of those suffixes, does it have the same meaning?
For example, 'polysemic' and 'polysemous'. Both from the noun 'polysemy'.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
In chemistry, such terms almost always have distinct meanings. For example, ferric refers to compounds of Fe³⁺ and ferrous to compounds of Fe²⁺, nitric oxide is NO and nitrous oxide is N₂O, and so on. (Note: I just thought of anhydrous/anhydric, which is an exception.) In other sciences, an -ic and an -ous form sometimes are synonyms (like achromous/achromic), and sometimes aren’t (like autonomic/autonomous).
Non-technical -ic/-ous or -ic/-ious pairs are often, but not always, synonyms (like euphoric/euphorious). There are also some words whose final syllable coincidentally happens to be -ous or -ic and also happen to have a corresponding -ic or -ous word, such as arsenic/arsenious or arsenous, and generic/generous. (Thanks to other users for sharing better examples!)
No, they need not have the same meaning. For example, "generous" and "generic" share a root but have very different meanings (from M-W) despite bearing those suffixes:
generous: "liberal in giving : OPENHANDED" (among other meanings)
generic: "relating to or characteristic of a whole group or class : GENERAL" (among other meanings)
Another answer gives good examples from chemistry.