Neither will be seen in the club's colours this season, though, as Watford recruit young talent for the future.
Penaranda, La Liga club Granada's youngest ever first-team player at 18, agreed a four-and-a-half year contract.
However he will stay with the Spanish side -- whose Italian owner Giampaolo Pozzo is the father of Watford owner Gino -- on loan until the end of the season.
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He has scored four goals in 13 appearances for the club, including two against Levante which made him the youngest overseas player to score a brace in La Liga.
France under-21 midfielder Doucoure signed a similar length contract and was immediately loaned out to Granada for the rest of the season.
No fee was disclosed for either signing, although reports have suggested Doucoure could cost Watford up to eight million pounds ($11.55 million).
"The opportunity to sign Abdoulaye Doucoure now was one we wanted to act upon, given our long-standing interest in the player," Watford CEO Scott Duxbury told the club's website (www.watfordfc.com).
"He is very much part of Watford's future plans, and in the time between now and pre-season he'll be experiencing high-level competitive football in Spain."
Doucoure, a product of Rennes' youth system, made his debut for the club's first team during the 2012-13 campaign and has made more than 70 appearances.
(Reporting by Martyn Herman, editing by Alan Baldwin)
