Canonical sources say that Darth Maul actually survived after being defeated by Obi-Wan despite being cut in half.
Since Darth Maul survived, and Palpatine took in Anakin as an apprentice, did the Emperor actually have two apprentices at the same time? Did Darth Maul stop being Palapatine's apprentice after his encounter with Obi-Wan?
"Any master who instructs more than one apprentice in the ways of the dark side is a fool. In time the apprentices will unite their strengths and overthrow the master. It is inevitable; Axiomatic. That is why each master must have only one student." ―Darth Revan's holocron
We know Palpatine isn't a fool, so how can this be?