This can be a bit tricky. One of the things you will want to hold off for is to do this on a longer boss battle, such as the Battle of Fate in Phase 3 for instance. If you've dived into one of the girl's Cosmospheres deep enough, you will be able to triple and even quadruple purge depending on the number of Beats and the Burst rate. By Phase 3, for example, I ended up usually needed to purge after a few Beats. You can always count the number of Beats it takes, or just hold back on Purging and wait until your Burst is certainly high. Hold three buttons and shake the Six-axis controller in hopes of seeing it work at once. I wouldn't suggest making a habit of dual-, triple-, or quad-purging as miscalculating the Burst/Beat of the song can end it prematurely, losing your benefits, and making you have to start all over again.