It's not quite accurate to say there are only two kinds of cohesive devices. Instead, there are several types of cohesive devices, which can be grouped into broader categories. Here are two common ways to categorize cohesive devices:

1. By Function:

* Reference: Using words that refer back to something already mentioned (e.g., pronouns, demonstratives, synonyms)

* Substitution: Replacing a noun phrase with a shorter word or phrase (e.g., "do" for "do that")

* Ellipsis: Leaving out words that can be understood from the context (e.g., "I have two apples, and you have three.")

* Conjunction: Using words or phrases that connect sentences or clauses (e.g., "and", "but", "because")

* Lexical Cohesion: Using words that are related in meaning (e.g., synonyms, antonyms, hyponyms)

2. By Type:

* Grammatical Cohesion: Focusing on the grammatical features that create connections between sentences (e.g., pronouns, conjunctions, ellipsis)

* Lexical Cohesion: Focusing on the vocabulary choices that create connections (e.g., synonyms, antonyms, repetition)

It's important to note that these categories are not mutually exclusive. For example, pronouns can be considered both a type of reference and a grammatical cohesive device.

Ultimately, the best way to understand cohesive devices is to see how they work in context and analyze how they contribute to the overall coherence and clarity of a text.