"I practice the piano and the guitar."
From what I've seen, 習う is usually used when talking about learning a skill from a teacher, whereas 練習する is used when talking about practising a skill in order to become more adept in it regardless of whether it's from a teacher or the subject themself.
Feel free to correct me if I'm mistaken, as I myself am not a native Japanese speaker.
Wouldn't it be 練習しています? Because this sentence is saying that they do this regularly and the ていす form also has the meaning of doing something regularly and not just in the moment. For example, when saying what you're studying in school it would be:
ジャナリスメを勉強しています. Because this is something that is still in progress and we are working on it.