"Two are better than one" or "Two is better than one"?

TL;DR: Usually choose are.

The question asks which of these two “is actually correct”:

1. Two is better than one.
2. Two are better than one.

Unfortunately, there can be no answer to that question. It’s a leading question. The problem is that the question by its nature forces the answerer to concede that only one of them is “correct”, necessarily leaving the other in some “incorrect” category.

But that isn’t how English works: English isn’t a multiple choice quiz with one “correct” answer. Both versions occur in print by native speakers, and so both versions (can) have their place.

The clearest case for choosing the singular is when one is talking about the numbers themselves, such as saying:

• Two is greater than one.

Then you really must use the singular form, since you are really just saying that 2 > 1 — or in other words:

• The number two is greater than the number one.

However, in most other circumstances, you are talking about two somethings, and two somethings are more likely to take a plural verb than a singular verb.

• I just read a series of three books, and I find that the first two are better than the third.

If you look at historical usage, the singular version was virtually unknown until comparatively recently, at least in this Google N-gram:

You do still have to dive into the actual citations for the two is case and the two are case separately.

The singular choice is nearly unknown in the 1800s, although there is this example dating from the Annual Register of 1800:

I have seen that two is better than one, and that a threefold cord is not easily broken, and have therefore cultivated friendship with much zeal and disinterested tenderness: but I have found this also vanity and vexation of spirit, though it be of the best and noble sort.

That is example of nominal agreement, where the collection of two together is the thing being considered.

In contrast, the far more numerous plural examples are thinking of two items separately. Certainly when the number is just a cardinal number used in a noun phase, like saying two people are at the door, then there is never any question of the plural being the only grammatical choice.

However, it is really only in the last 50 years that the singular version has caught on much. Also, if you change the search to look only in (allegedly) British sources versus (allegedly) American sources, you will find that the British sources have a higher ratio of plural to singular than the American ones have. Note however that in both cases, the plural is still more common.

If you look at the Corpus of Contemporary American English 1990–2012, it has 3 citations of the singular two is better than one (2 spoken and 1 fiction) compared with 2 citations of the plural two are better than one (1 spoken and 1 fiction). That isn’t really much to go on. If you relax the constraint by dropping the than one part, then there are 7 citations for the singular and 4 for the plural.

If it were me, I would in most cases go along with most writers not just of yesteryear but also of today and choose the plural version just like they did.

That is the best I can do for answering the leading question of which one is “actually correct”: a long exposition demonstrating that the question is itself flawed, and that while both can have their place, usually the plural version works better.