In a test dummy scenario or against a group of mobs with lots of HP, infinite flares are superior. However, Fire II is important if it can get mob HP low enough to not need another cycle of Transpose > Flare x2.

When starting from a full MP bar with Raging Strikes up, you should throw in enough Fire II's so that you can finish the pack with Flare > Convert > SC Flare. Once that initial bar of MP is gone, you should do either Transpose > Flare > Flare or Transpose > Flare > Fire II > Flare depending on remaining mob HP.

I've had a lot of cases where all the mobs have a sliver of health left and both BLMs are standing around doing absolutely nothing for 5 seconds trying to time the MP tick for double flare. This could have been avoided if they had used an extra Fire II before casting Flare.