You should do the math if you are going to tell me to look up the numbers. Overload can get an 8x multiplier against barriers. That is why it does more barrier damage than Reave.
Also you aren't killing an Insanity Atlas in 5s w/ Dark Channel and Reave DOT. Both have a 0.5x multiplier to shields, neither have a detonate evolutions. But for fun, let's see what the damage is from Kaidan and Javik w/ appropriate leveling:
Reave max damage (Duration, Either, Barrier + Armor):
Base DPS: 70
Power Bonuses: 0.25 Power Synergy, 0.45 Alliance Officer, 0.3 Alliance Officer; 0.1 Javik Squad Bonus, 1.1 total
Total DPS: 147
Duration: 4 base, 7 with Rank 4, 7.4 w/ Javik Squad bonus
Barrier Mult: 2x base, 2.75 w/ Rank 6 B+A
Armor Mult: 1.5x base, 2.25x w/ Rank 6 B+A
Shield Mult: 0.5x
Dark Channel (Duration, Either, Pierce):
Base DPS: 20
Power Bonus: 0.2 Rank 3, 1.4 Vengeful Ancient, 0.1 from Kaidan; total 1.7
Total DPS: 54
Base Duration: 30
Duration Bonus: 0.4 Rank 4
Total Duration: 42
Barrier Mult: 2x, 2.75x w/ Pierce
Armor Mult: 1.5x, 2.25x w/ Pierce
Shield Mult: 0.5x
6+6 Biotic Explosion, Insanity:
Base = 100 * (1 + (12 - 2) * 0.15) * 3.375 = 843.75
BE Shield multiplier: 1x
Insanity Atlas (Level 60): 16875 shields, 16875 armor
Apply BE:
16031.25 shields, 16875 armor
Reave shield damage: 147*7.4*0.5 = 543.9
Dark Channel shield damage: 54*42*0.5 = 1134
So after 42s you have an Atlas with 14353 shields left.
An Atlas at Level 1 only has ~11,000 shields, but if you are at level one your combo has a lower difficulty multiplier.
If you meant after the shields are down, which is radically different since you already dealt with half of the Atlas's total HP, then you can get damage like:
6+6 BE Armor: 1687.5
Total Reave Armor damage = 147*7.4*2.75 = 2991.45
Dark Channel Armor damage = 54*42*2.75 = 6237
Total Armor Damage: 10915.95
Which is still less than 11250 of a Level 1 Atlas, and takes 42s to see.