Here's the email I received back from one of the keepers at the Oregon Zoo which I thought was quite funny since I saw Don Ho perform in Honolulu while I was there:
"Mukenko" is the larger one, also known as "Poppy". Personally, I've always thought that was a dumb name for a hippo, so I took to calling her "Tiny" instead. There were two reasons for this: the incongruity of naming the larger one "Tiny", and the fact that it lends a certain "Don Ho" flavor to the area. "Tiny Bubbles...in the zoo..."
So, "Mukenko" is also known as "Poppy" who is also known as "Tiny" leaving "Kiboko" with the nickname of "Bubbles."