training for speed with a double end bag?

I am looking at purchasing a double end bag, but have never used one. I train in some kickboxing, so I was considering getting one with two bags connected, for a bit more versatility. I have no idea what to expect since i have never trained with one and my instructor has never used one with two bags on it so he cant really give me any advice, but I have heard that the extra bag slows it down a bit and my main reason for wanting one is to help me on my speed. if anyone could send me any links to anything regarding double end bag training that would be great. below is a link of the two bags I am looking at getting.

http://www.ringside.com/DETAIL.ASPX?ID=24687

http://www.ringside.com/DETAIL.ASPX?ID=24607