Our daughter gave us this one as a Christmas gift for our January trip to FL:
http://www.amazon.com/Dual-Bike-cover-transport-bikes/dp/B000WENDUS/ref=sr_1_1?ie=UTF8&qid=1399656490&sr=8-1&keywords=bike+cover+waterproof+2+bikes+for+bike+rack
And all the way to Florida and back, it kept our bikes clean, dry, and road-salt-free. It's SO waterproof, that a puddle formed between the 2 bikes: luckily I saw it before it dumped a gallon of water on my shoes....LOL!! VERY happy with it - especially after we watched a man from Ohio trying to get his 2 non-covered bikes to pedal again ("frozen" with road salt)