1-a. She doesn't sing as well as me.
1-b. She doesn't sing as well as I do.
In 1-a, an object pronoun is used after 'as', it is an informal style, whereas in 1-b, subject+verb is used after 'as', it is more formal.
As you can see, in your example #2
#2. I don't like him as much as you.
2-a. I don't like him as much as you do. ( 'you' is a subject)
2-b. I don't like him as much as you. ('you' is an object pronoun)
2-a and 2-b are possible variants of #2. Have you noticed that we have a subject 'you' and an object 'you'?
In 2-a, both of us like him, but I like him less and you like him more. Ambuguity is cleared out in 2-a because 'you' is a subject.
What are the two equal things you compare now?
1. I don't like him. (subject)
2. You don't like him. (subject)
In 2-b, I don't think it's usual to regard 'you' as a subject. For example, I don't like him as much as she. In this sentence, it is ungrammatical. You either say I don't like him as much as her or I don't like him as much as she does. So I don't think 2-b is that ambiguous. It means I like you more than I like him. 'You' is for sure an object of the main verb 'like'.
What are the two equal things you compare now?
1. I like him. (object)
2. I like you. (object)
This is your question,
I don't like him as much as you don't like him. :?:
==> I think this sentence is gramatically strange. 'As + Adj/Adv + as' is used to compare two equal things. Here in your sentence, you compare the feeling of 'like him'.
I don't [like him as much as you like him.]
==>'don't' negates the idea in my brackets. Is this the negation scope you need? Therefore, we apply 'pro-verb' substitution to the latter 'like him', and then it generates a new variants,
I don't [like him as much as you do.]
So the latter 'like him' is within negation scope, I think there is no need to negate the latter 'like him' again.
What do you think?
:wink: