Depends on the fight. If you only have enough time to get off one more round of CD's, go ahead and pop them together. If you're potentially gaining more CD's used by using them on CD, such as on LOD, go ahead and pop them off CD.
Honestly, the only CD you benefit from casting with Bloodlust is Mirror Images. Stacking haste cooldowns provides no benefit over using them separately, and like I mentioned earlier, combustion only benefits a limited number of spells. If you're DPSing the boss during the first transition phase (and you should be), you should aim to pop your second Icy Veins while attacking him here, as correctly specced mages have should be permanently critting here.