その二人はすばらしい夫婦になるだろう。

English Translation

Those two make a fine couple.

夫婦 includes only “married couples”. But “couple” in English also includes “couples in a relationship” in general.

I also find すばらしい as for “fine” a bit unnatural because 素晴らしい夫婦 means “a best-in-class couple” or “a successful couple” (i.e. both of the two will be socially achievable and they will be better than other couples in the world). お似合いの(おにあいの)sounds much better because it merely means “a good match”. They don’t have to be superior to other couples in terms of wealth or social status. They will be simply happily married.