Your numbers are way off. Bone dragon drains 300 energy from each hero, then at least 175 per second for 6 seconds (1050 total), assuming no expertise bonus. Dragonslayer blocks 1600 energy, assuming no expertise bonus. Expertise adds 10 per point to DragonSlayer's standard, and 1.4 per point to Bone's per-attack drain.
How they should work together (according to DSSven):
Bone drains 300*5=1500 energy at the start of combat, which is almost enough to drain the banner. The banner is completely drained in the next second (875 more energy). A DS with maxed expertise against a bone with no extra expertise might last until the third second of combat.
How they do work together:
Bone drains 300 + 875 = 1375 energy in the first 6 seconds, which is not enough to destroy the banner without expertise. Drain is counted as one chunk, and not multiplied by the number of heroes.