Hi.
Something I was taught a few years back by a guy who I really respected as a solid drummer was the following: When you first get a dbl kick (and for the sake of this reply I'll assume we are dealing with a right handed player) practice single kick patterns with your left foot. So, if you like to play along with something pretty easy (at first) do all the work that you would normally reserve your right foot with your left. It sounds stupid, I know, but what it does is help you to have a left foot capable of doing more than just mirroring 16th notes or hi-hat work. Your foot has to be taught, just like the other one and by playing single bass parts with your new foot on double kick, you'll help build more than just a little dexterity. If you are right handed, you'll have to find a setting for your hi-hat that you can deal with (as you won't be able to use your right foot to cross over the left to control it) but it is not as easy as it sounds if you've never done it. I also believe firmly that playing slowly is every bit as hard as playing really fast. I am in no way knocking speed, but if you take a really fast fill and slow it way down, you'll find that your set becomes a very good truth detector in that any mistakes you make will be easily observed.
I hope this helps. There are some other great suggestions here but I thought I would give me 02.
Keep Stickin
Best,
Isaacs