I was not involved but:
No 6000 foot rule in Canada;
Departures were IFR using pilot applied visual departure separation;
It is the norm to start issuing the subsequent take off clearance before the preceding is airborne timing the "cleared for take off" as the nose wheel lifts. In this case the timing was a little off and the B77W (very light going the 45 minutes to Montreal) was quickly out of the blocks and the E190 rejected from a high speed;
I doubt both aircraft "accelerated... at same time."
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Not trying to shift blame but why did it take so long for the B77W to realize that the E190 that they were visual with wasn't in the air where it was supposed to be?