First things first. If you have the genetic gift of a extreme double bicep peak you can thank the BB'ing Gods. But if you were born with a somewhat flatter shaped bicep, chances are any degree of a respectable bicep peak will never happen. All about the muscle inserts of your body and their limits on the structure of muscle. You can certainly improve the muscle size of the biceps greatly, having 19"-20" guns hanging by your sides, many guys have. Just my view, may be wrong, but from watching PL'ers and Oly'ers, guys with longer, flatter biceps seem to be the strongest.
Good bicep exercises that people don't seem to do much anymore.
1) Incline DB curls. Lying on a incline bench, have the thumbs facing forward. As curling up, twist the hands/wrist inward. At the top position the thumbs should be pointing outwardly. Try to cramp/flex the biceps for a second, lower the DB's and repeat the exercise. Keep the head on the incline board at all times.
2) Bent forward curls. Holding a BB, with a close grip, bend over to about a 45 degree angle. Curl up the bar and touch the forehead each rep. Should feel a strong cramp/flex in the biceps. Lower and repeat.Want the elbows out and forward a bit during this movement. Can do this with a DB, one arm at a time.
3) Concentration Curl. Simple basic exercise which has built a lot biceps over the years. The full stretch and full contraction throughout the movement. Keep the elbow wedged against the inner thigh at all times, near the knee. Can also do this with a BB.....close grip.
3) Drag Curls. As the name applies, drag against the legs to around upper ab area. Keeps the tension on thee biceps throughout.
4) Close grip chin. As the name applies. Chin to the the upper chest and hold for a second. Can do this on a lat machine, as second choice.
Cable work has an important part in trying to get that bicep peak also. Draws upon some bicep (and surrounding area) fibers not usually called upon by standard free weights, even machines.
As far as preacher curls go, may want to find a design close to the original Scott/Gironda one. And using a straight bar. Of course DB's on occasion. On any version, the placement of the elbows are of prime importance. Good Luck.