They decided to get married last week although they ... each other for only five months. (have known - had known - know - would know)
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The knowing took place before the decision, so the past perfect should be used: had known. That is how I would answer it in the absence of any more context.
However, a native speaker might use the present perfect if they know that the decision was made but the marriage didn’t take place yet (as if the sentence were “They’re going to get married although they have known each other for only five months”). So it’s possible that you would hear such a construction from someone.

The barber cut Ahmed's hair
Use:had
Use :was