Characters sometimes have multiple meanings, so when breaking words into characters, we need to figure out which character meanings are pertinent. Anyway, let's take a look...
知
知 is both a noun and a verb:
When used as a verb, 知 is used to indicate you know or understand something, such as in:
我知道那个人。 I know that person.
他不知花了多少时间读《离骚》。 He doesn't know how long he spent reading Lisao. (source)
人际冲突使我不知所措。 I don't know what to do when people argue. (source)
When used as a noun, 知 basically just means 知识 = "knowledge". This is how it's used in:
据我所知，她在市里有座房子。 To my knowledge, she has a house in the city. (source)
我们已经通知了直系亲属。 We've already notified the next of kin. (source) Here, 通知 = "notify" or more literally translated "communicate knowledge"
It's not common in modern Mandarin to use 知 as a standalone noun outside of fixed phrases, but you see it in e.g. Confucius quotes:
知之为知之，不知为不知，是知也。
When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it - this is knowledge (Wiktionary)
识
识 can function as a verb and a noun:
As a verb, 识 means "to recognize" or "to know", such as in:
我不识字。 I'm illiterate, or more literally "I don't recognize characters".
人脸识别系统 Facial recognition system, or more literally "human-face recognize-as-distinct system".
我认识他。 I am acquainted with him.
When used as a noun, 识 also means something like 知识 = "knowledge". This is how it's used in:
缺乏常识的人能意识到自己缺乏常识吗？ Do people who lack common sense realize they lack common sense? (source)
我们的教授学识渊博。 Our professor is erudite. (source: Pleco) Here 学识 is something like "academic knowledge".
知识
知识 can only function as a noun, and is basically the same as "knowledge" in English. The 知 and the 识 both mean 知识.
您对环保的知识通常从哪里获取？ Where do you normally gain knowledge about environmentalism? (source)
知感
What your reading is native literature (albiet simplified for the textbook) by the author 梁晓声; you're going to encounter more figurative speech. In any case, the author defines what he means by this word here:
所谓知感就是别人呈现给你，展现给你，说给你听，要求你记住的那一部分。
So-called 知感 is what other people present to you, reveal to you, say to you, and require you to remember.
So that's the what the author means by 知感. I'd treat this as a "local" definition, so you won't see it outside this context.
He's saying 知识 is made up of 知 and 识, where 知 represents knowing things, and 识 represents 思考 or considering what that knowledge means. He's emphasizing the importance of 思考ing. He's speaking figuratively, so it doesn't have to be etymologically correct.
“知识” 两个字我始终认为它是要分开来谈的，……
Especially this part: 分开来谈。For example, about the 来, according to wiktionary it can mean "Meaningless particle for rhythmic purposes". ??
I believe that's correct. The 来 doesn't really have any meaning; it's used because the author felt it sounds better with the extra syllable. Anyway, you got a detailed answer here.
Also, I don't understand the combination of 两个字 ... 它。
It looks like you see a mismatch between the usage of 它 (singular) with 两个字 (plural). I think you're right, in that 它 generally refers to a singular object.
If we compare it with this version, this issue doesn't arise:
因此我跟我的同事说到，对于大学课程，其实我反对的主要是在方式上灌输。因为我历来认为知识两个字是要分开来谈的，知就是知感，识就是认识。
so it's possible this issue arose through simplifying the original text for the HSK Standard Course textbook.
I'm studying the HSK6 Standard Course, and I often find weird phrasings in the main text. When I'm able to compare it with the original text, I find the places where I feel "that sounds really weird" are usually those where the original has been edited; it's often because the author had to cram in all the HSK6 vocabulary and grammar somewhere. If you can find the original texts, the writing is usually much better (although unsimplified).
