What's the difference between them.
"What are you up to tomorrow? " and "What are you doing tomorrow?"
What's the difference?
also which is more common use?
If I want to ask my friends for hanging out and don't know they are free or not. How can I say? How can I ask them?? I want to know some phrase.
Thank you :)

Question

What's the difference between them.
"What are you up to tomorrow? " and "What are you doing tomorrow?" 
What's the difference? 
also which is more common use? 
If I want to ask my friends for hanging out and don't know they are free or not. How can I say? How can I ask them?? I want to know some phrase.
Thank you :)

Answer

No difference in my opinion.

But i would agree that "what are you up tomorrow" is a bit more friendly and also quite a fluent/natural phrase.

Answer

Hello!
If they are your friends, using 'what are you up to tomorrow' is a friendly way to ask them! They also have the chance to say they are busy tomorrow without causing you distress ( they could down a direct invitation). It can also help your friend feel good--you want to know what is going on in their life! 'What are you doing tomorrow' is more direct, so if you feel like being direct (like asking them to a movie) you can use this instead. But, I think the first option is much more friendly.

Hope it helps :)