If you want an old-fashioned way to talk about two things, use the noun twain. You might lament that your pair of dogs was divided in twain when you had to give one of them away.
The word twain is hardly ever used these days, so you're most likely to see it in an old book of poetry, or in the phrase "never the twain shall meet." This saying means that two people — or groups of people — are so unlike each other that they'll never manage to see things the same way. The origin of twain is the Old English word for two, twegen.