"Miikka and Joni know how to tango."
Translation:Miikka ja Joni osaavat tanssia tangoa.
You can actually say "Osaan tangoa." but "tangoa" isn't a verb there, it's tango in partitive. Compare it with "Osaan suomea.", suomea in partitive.
Either way I've rarely heard it osaa tanssija tangoa ive mainly heard osaa tangoa
Tangoa is the partitive form of the noun tango. Such a noun can follow osaavat
As for the verb, this site gives infinitive tangota. And following the pattern of a type four verb, 3rd person singular tangoaa, not tangoo, past tense tangosi, not tangoi.
Here, a Finn translates 'you tango' with tangoat, which would have the infinitive tangota. Duolingo accepts tangoat.
The infinitive of 'to tango' is tangota. So more literally, "Miikka and Joni know how to tango" is Miikka ja Joni osaavat tangota.
"Miikka and Joni know how to dance the tango" is Miikka ja Joni osaavat tanssia tangoa, using the partitive noun tangoa as the object of the infinitive verb tanssia.
I don't know which construction is more used in Finnish, but I've noticed that the verb tangota has yet to get an entry in Wiktionary, and that Google translate doesn't seem to like it.