They left the hall quickly, wanting to unwrap the broomstick in private before their first class, but halfway across the entrance hall they found the way upstairs barred by Crabbe and Goyle. Malfoy seized the package from Harry and felt it.
(Harry Potter and the Sorcerer's Stone)
When the verb seize is combined with from, does it mean the broomstick left Harry’s hand or do both the boys hold it?
Yes - seized here means took [violently, by force]. And from can be read as away from, which implies Harry no longer had "possession" (he wasn't still holding it, since it was taken away).
To seize means to take. To take something from someone is to remove it from their possession and place it into yours; to take it away from them. So yes, Malfoy took the broom away from Harry, and now Malfoy is holding it and Harry is not.
