Here's a decent doubles practice routine I've been using for the last few months. It has a couple of advantages over Bob's 27 in my opinion ; the doubles have to be hit as they would in a match with near misses causing a switch to another finish, and also it doesn't require any chalking.
You have two throws of three darts at each target. The target is actually the finishing score rather than just the double itself, so for example splitting the 20's sees you switching to 10's then 5's etc. This gets you nicely around the whole board, needing single segments and other doubles as you play through the game.
Hitting a target with the 1st or 2nd dart gives you a free throw at the next target which doesn't count as one of your two throws if you miss.
I like to use all three darts when practicing, so totally messing up a target and busting can be rescued by hitting a 50 Bull, giving you all three darts again.
Take the targets in any order, but I find top, bottom, left, right, the diagonals works well. It's game over if you fail to hit a target within the two throws, so it's back to the 20's for the order I take them in. The game is completed successfully when you hit the final target within the two throws.