First, I don't believe at all in training in two or more arts until you have attained 4th Dan in at least one. Practicing one style seriously consumes a great deal of your physical and mental energy. And now you want to try two? Not to say that having an understanding of elements of other styles after BB isn't helpful. It is, but it is your Instructor's job to show you these elements.
Second, I would not recommend learning another style similar to your main style (i.e. Tae Kwon Do to Tang Soo Do or Karate). Why? Because when you learn TKD or TSD or Shotokan you are training your body to move, twist, punch, kick, block, and think a certain way. A kick is NOt a kick, and a block is NOT a block regardless of where you learn it. Each style is unique. Pick one style's approach to technique, but stay with that style. Otherwise, your body will be forced to learn two different, and not usually complimentary, versions of technique. Additionally, if you are studying a similar style, you will always be comparing what they do with what you are used to doing.
Third, new style new rank. No exceptions. It doesn't matter if you are a Master Instructor in Tae Kwon Do. You want to start Kenpo, TSD, or Shotokan? Put on your new white belt and leave your attitude and ego at the door. I began aikido training previously because I had reached 4th Dan in TKD and wanted to try something new. TKD is the only striking art I want to practice, so I decided to try a soft style to avoid any confusion.
These guys who claim black belts in 3 or more arts make me laugh. You are only fooling yourself. How good are they really going to be?