Luke 11 concerns demons. Its not Satan himself.
14 And He was casting out a demon, and it was mute; when the demon had gone out, the mute man spoke; and the crowds were amazed. 15 But some of them said, He casts out demons by Beelzebul, the ruler of the demons. 16 Others, to test Him, were demanding of Him a sign from heaven.
Now... if you look at Matthew 12, you will see that the casting out of Satan is a synonymous term with casting out of actual demons, not Satan.
22 Then a demon-possessed man who was blind and mute was brought to Jesus, and He healed him, so that the mute man spoke and saw. 23 All the crowds were amazed, and were saying, This man cannot be the Son of David, can he? 24 But when the Pharisees heard this, they said, This man casts out demons only by Beelzebul the ruler of the demons.
25 And knowing their thoughts Jesus said to them, Any kingdom divided against itself is laid waste; and any city or house divided against itself will not stand. 26 If Satan casts out Satan, he is divided against himself; how then will his kingdom stand? 27 If I by Beelzebul cast out demons, by whom do your sons cast them out? For this reason they will be your judges. 28 But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you. 29 Or how can anyone enter the strong mans house and carry off his property, unless he first binds the strong man? And then he will plunder his house.