"Two angry dogs are growling."
Translation:Kaksi vihaista koiraa murisee.
It would still be in the singular, although I wouldn't recommend using the kaksi paria koiria structure in the subject position with dogs and other things that do not usually come in pairs (unlike, say, socks). It sounds very clumsy. Try kaksi koiraparia or find some other way to express the same thing. It sounds perfectly natural as an object or a predicative though. :)
The plural nominative form is indeed "koirat". However, after numerals other than "yksi" you need the singular partitive, "koiraa".
"Koira murisee." - the dog growls/is growling
"Yksi koira murisee." - one dog growls/is growling
"Koirat murisevat." - (the) dogs growl/are growling
"Kaksi koiraa murisee." - two dogs growl/are growling