To me, "of them" comes across as slightly wrong, because it's partitive. Consider:
- "There were six apples on the table; Susie ate two of them."
- "Susie eats apples often. This morning, she ate two."
A quantity of something implies part of a whole: Susie ate a part of the whole set of six apples. But in the second sentence, which isn't partitive, one would not ordinarily say "she ate two of them".
Another example:
- "My two sisters go to church every Sunday."
- "Two of my sisters go to church every Sunday."
The of construction in the second sentence clearly suggests that I have more sisters than two, whereas its absence in the first clearly indicates that I have only two sisters.
Similarly with your favorite colors, or the cars of your dreams: you have a fixed quantity, and you're speaking of the entire set. So you might say "Two of my dream cars are a BMW and a Mercedes" if you mean that you also dream about a Porsche and a Jaguar. However, if the first two are the only ones you dream about, you'd simply say "I have two dream cars, a BMW and a Mercedes." Therefore, when answering the hypothetical question, the elliptical form to use would be simply "I have two," not "I have two of them."
As for "there are two": it's better than either of the "of them" alternatives, and there isn't all that much to choose between it and "I have two."
An idiom that could be useful here is that of the double-edged sword, which indicates that something positive and useful in one scenario can be negative in another, and may even be used against you. A double-edged sword cuts both ways. The idiom is typically used in situations where the negatives are not obvious, and may come as a surprise.
Working with the example, you could say something like this: "For all of its advantages, ABC is something of a double-edged sword." Then proceed by describing disadvantages of ABC.
Best Answer
'Two sides of the same coin' does not quite mean what you describe.
It essentially means that two things are the same. I might use it in a context where someone is describing someone else as both 'lazy' and 'messy' and in response I could say that those are 'two sides of the same coin' and thereby suggesting that they are one and the same, in this case suggesting that the person is messy because they are lazy.
As far as I can tell you want a pithy phrase to describe two things that are good together but not necessarily either good or as good apart.
I'd suggest a simile in this situation. A very common type of phrase is to say:
Currently on the London Underground for example there is an advert that says something along the lines of 'a woman's hair without product x is like rock without roll' (the latter part usually a ridiculous separation for humour value). There's no standard phrase used here but it's an opportunity to be creative.
A couple more examples: