I don't believe you can have two auto-increment fields in a table, and it looks like the 'id' field is set to auto-increment.
I think your'e going to have to
a) query the table to see if the user_id for your new entry already exists
SELECT user_id,sub_tile_id FROM table WHERE user_id = [user id] ORDER BY sub_tile_id DESC LIMIT 1
(read the entry from the table with the same user_id as your new entry, and the highest sub_tile_id value)
b)if it does, read the highest sub_tile_id, increment it by one and use this value to insert a new row.
c) if the user_id does not exist, insert a new record with the new user_id and the sub_tile_id set to 1.