I have a visitor's visa valid for another 5 years and I want to do a 2 year specialty (dentistry). Will I end up with a 2 year visa only? Even if I have a multi- entry visa for another 5 years? Or will I be able to keep both??

If you apply for another visa/status, I think you lose the previous one.

You may have more than one visa, as long as they are different types. You stated that you have presently a B2 (or perhaps a B1/B2) visa and are seeking what sounds like a J1 (exchange visitor) visa. You may have both without jeopardizing the first. What you could not have would be two valid B1/B2's, for example.
If you obtained the J1 visa for the specialty program you wish to participate in, you would still retain the B1/B2 so you could travel in the future as a tourist. When admitted as a J1 visitor, your passport would be stamped "D/S" which means "duration of status"; a time consistent with the length of the program. When you were (or are) admitted as a tourist, your passport is stamped (usually) something like "admitted until _____) with the date marked on the line. Not to worry.