I use the multiple attempts option (two attempts), and my students get a random selection of questions from my question bank for both attempts. Of course, by chance, some of the questions will be the same due to the random selection process, but the more questions you have in the question bank, the less likely the student is to get the same question on two attempts. Most of my question banks have about 50 questions for quizzes, and the quiz is set up to select 20 questions from that question bank. In my case, students do get a few of the same questions on their second attempt. 

I also have it set so that the students can only see which questions they earned points for after they complete the quiz. It does not show them the correct answer or the other answer options they had to choose from.  They can still tell from the points earned whether or not they got the question right or wrong. After the deadline for the quiz has passed, then they can see all of the answer choices along with the correct answer choice and the answer that they selected.

You can try what you are already doing and check to see if it meets your expectations.  Keep the settings you have for the first quiz. When students have finished taking it twice, select their score from the gradebook and then select Speed Grader.  Their second attempt on the quiz will appear.  From that screen, off to the upper right portion of the page, you will see the date and time they they completed their first attempt.  You can select than and view their first attempt and see how many questions they had on the two quiz attempts that were the same.

As an alternative to your current setup, you could split each question bank for a quiz into two separate question banks and have a link for two separate quizzes with a single attempt each. That way, students first attempt would involve questions from the first question bank, but their second attempt would involve the different questions that you placed in the second question bank.  But, if you only want to keep their highest score, then you would have to go through the grade book and select the lower score for each quiz for each student and then select the "excused" option for that score.  That could be pretty laborious.