I took my boy/girl twins to the Pirate's League a month after they turned 4 (in 2010).
We did the basic packages, though my daughter did the girl version of the regular pirate package (not sure the name). She got glittery makeup and nail polish.
She really enjoyed it. My son was tired and needed a nap so he did not enjoy it as much, plus his pirate didn't really put a lot of effort into getting him to participate. He didn't like the feel of the makeup on his face, either (something I probably should have tested on him ahead of time).
My daughter's lady pirate was very good.
They both enjoyed the naming ceremony, getting their swords and going into the secret room more than the actual make up part.
I do have the best pictures from inside the room. They both had fun posing, and then, when we were getting the photos, they each wanted a bag of gems, so that was the highlight for them, too. And they played with the sword, sash and coins when they got home for months afterward.
I'd say as long as you can pick a good time when your twins aren't tired, they will really enjoy it. I would test to see how they like make up being applied first, just to be sure they are prepared for the experience.
My decision to do Pirate's league was really cemented by the fact my daughter did not like having her hair brushed so BBB was out of the questions. And this way, the twins could do something together and one wasn't left out. Really a good choice for b/g twins or brothers and sisters close in age.