Ice Nova will give a more reliable performance on single target (the DPS is theoretically pretty close but Ice Nova is just far easier to maximise), but you should still consider using Frost Bomb on Tectus, Twin Ogron, and any other fight where you feel like you may have two or more targets available next to each other for a significant portion of the fight.
It's so easy to get two fingers of frost procs at once that there is no benefit to holding one, even with Frost Bomb. Especially if Water Jet or Frozen Orb are up.
What does having two before casting Frost Bomb even accomplish for you? You don't gain anything by casting two FoF's into it back to back over casting it and then one and then proccing another. It's duration when cast is long enough that you are unlikely to save any GCDs by waiting, and you are likely to lose potential ice lances, which will cost you more DPS.