Here are some links to some info on twin keels:
Roll Attenuation and Bilge Keels
Why Twin Keels, by John Letcher —site by Nels Tomlinson
Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels
I've owned two twin keel boats over the years. The first was a Westerly 25 built in 1967. The drag is increased and they can't point quite as well as a single keel boat. The second twin keel boat I owned was a 35" custom design that was bought from me by her designer and some changes made, most notably the change from single rudder to twin sailing rudders and a centreline rudder for powering. After these modifications she was sailed from Victoria to New Zealand and back by her designer and performed admirably. Quite an interesting boat in many respects. Here's a link to her listing as she is currently for sale:
Vela Yacht Sales (Victoria, BC)
Brian