As a translator (mainly localization), I want to say that depending on your purpose, there are places where zh-tw "Chinese (Taiwan)" and zh-cn "Chinese (China)" are and should be treated as completely separate languages. Although in these places, you would also treat en-US and en-UK as different languages.

Things that differ include, for example:

• Punctuation: zh-cn is aligned to the bottom-left and often the use of English double quotes are accepted, while zh-tw is always center aligned and in general doesn't use English quotes.
• There are major differences in phrases and sentence structure, especially in formal text, government text, and colloquial language.
• Vocabulary can vary from minimally to wildly. For example, the zn-cn translation for "row" and "column" is opposite to zh-tw. Also, the units of measurements used can also differ depending on field and purpose.

For the purpose of conducting business or even research, as well as dialog, it's not a big problem to treat the two as the same language, because you can generally count on the reader/listener to accommodate you.

But for the purpose of linguistic studies or translation, you should always treat the two as different languages.