You can't double dip. In fact, I've asked to switch a credit card from the BA program to AA (run by the same company), and the rep added me to AA but forgot to turn off BA. The net result was that I didn't get miles on either.
I wish I had glowing reviews about DALC, but unfortunately, I got horrible service from them over the years. Calling once to sign up a card was never enough: in my experience, it was necessary to verify at least once that the card you called in had indeed been registered (and with the right program). On at least two occasions, it hadn't been, and they refused to honor my card usage. The points post *very* late, or not at all. In addition to this, they have very few restaurants in Hawai'i, so we have just about given up on them.
That said, I found that many of the restaurants on the program are quite good, and, if you're travelling in a new city, the restaurant guide can be helpful in finding a decent place to eat.
[This message has been edited by honu (edited 02-09-2000).]