At your level I would concern myself with just training hard and eating a LOT. At 138lbs and 55lb curl, you aren't going to have very impressive biceps no matter what you do.
Just take a few years to work your body very hard. What kind of workouts are you doing? Full body? Split? Whatever it is, make sure you are really pushing the poundages on your lifts and eating like a monster.
Are you talking the split lengthwise or the split on the outer arm? What I've found is that doing brachialis work has brought that muscle out and it really adds a dimension of development to the outer arm.
If you mean the lengthwise split between the two heads of the biceps itself, I'd say you're going to need to be at a really low bf, that's all.
Of course since I've never been below 10% bf I'm probably not one to talk
i looked it up online and i found here on bodybuilding.com the article "getting the double headed monster" or somesort said to do these three excersizes
cable preacher curl, seated alternate curls and straight bar barbell curl.
i've been working out my biceps alot lateley and i'm starting to get the little indentation in my biceps!
The split you are talking about (like the one Boyer Coe has) is largely genetic.
From an interview with Boyer Coe:
"[ Q ] Interestingly you are known among other things for your split biceps. Have you always had this, or did you train for it?
Purely genetics. I remember I was about 16 when I realized I could make my biceps split while doing a pose. I was so proud. That evening, I told my Dad that I could make my biceps split and that no other bodybuilder could do that. He sat, watched me for a minute, and then said, "Let me see if I can do that." He rolled up his shirtsleeve and sure enough he could do the same thing. So it was purely genetic." http://www.bodybuilding.com/fun/drobson271.htm
The best way to acheive what you're looking for is done by simply adding muscle to the biceps. No special exercises or techniques will make it somehow split up.
But, I found straigh BB curls to be superior to EZ bar curls as they hit the inner head better (keep in mind all curling will hit the whole bicep though). Then hammer curls are good, as well as dumbbell curls.
But it's true most shape and specific definition comes from genetics. All you can do is add muscle and keep low bodyfat and your body will dictate the outcome.