<p>I am addressing the fact, it just seems to be going over your head. I will break it down for you.</p>
<p>Although (this right here tells you that what follows will be contradicted later on) the two kangaroos are very similar (OK, now we know that the next phrase(s) will show how they are not similar), one requires little water, while (this tells us that the next adjective should be an absolute, like "little", because "while" indicates that what follows should be contrary to "little") the other require the greater amount of water.</p>
<p>Hmmm. "The greater amount" is not contrary to "little". It is not logically equivalent to "a lot". "The greater amount" is logically DIFFERENT than "a lot".</p>
<p>Funny how I never once said it didn't sound right, you seem to be fabricating that yourself.</p>
<p>If you want to correct the sentence, you could say:
"Although the kangaroos are very similar, one requires little water while the other requires a lot of water." See how "a lot" is of the same adjectival category as little? "The Greater amount" is not equivalent to "little" or "a lot". Heck, I don't even think you could use "more". More does not exclude the possibility that they both require "little" water, in which case both would be similar, which makes the statement false.</p>
<p>You need to use absolute adjectives with absolute adjectives, and relative adjectives with relative adjectives.</p>
<p>Here. When broken down, the sentence has two parts:
Although the two kangaroos are similar, this kangaroo requires little water.
Although the two kangaroos are similar, this kangaroo requires the greater amount.</p>
<p>You could be discussing the same kangaroo.. However, if you use absolutes like "little" and "a lot", the two parts cannot exist together, allowing you to use the word "while":
Although the two kangaroos are similar, this kangaroo requires little water.
Although the two kangaroos are similar, this kangaroo requires a lot of water.</p>
<p>The two cannot be one in the same kangaroo. Therefore, you can compare the two in a logical sentence.</p>