A vacuum canister works the same way a tank works on an air compressor; it holds the air out of the compressor. If you have no air pressure out of the compressor it won't hold much. Like wise the vacuum accumulator depends upon the lowest gauge reading of manifold pressure. If you don't have much manifold vacuum due to cam overlap keeping the valves hanging open it can not collect much vacuum to work with the brake booster.
You can add an electric vacuum pump off of a diesel engine powered truck (diesel engines have the manifold open to the atmosphere so they have no vacuum), or off of an early Cadillac or Buick that had everything on the car (windows, antenna, seat, etc.) powered by vacuum from the motor.
If you have a SBC, even with tall valve covers, an eleven inch power brake booster will fit. The larger diameter offers more surface area for the vacuum to work against. I am surprised they had you buy the much more expensive dual small boosters instead of the more common and much cheaper stock booster. Only reason I can think of is they wanted your money.
One other thing to look is a vacuum leak in regards to the check valve in the vacuum supply line. It seals the brake booster from the manifold when the engine is off so that you can stop the car when you first crank it up. There should be enough vacuum retained in the booster to allow the brakes to work.
Finally there are two holes in the brake pedal. One is for power brakes and the other is for manual brakes. You might try moving it to see if the change helps or hurts (I have no idea where it is set now).
I have to pull these suggestions out of the air because I am not there to see what is going on. A vacuum gauge reading from your engine idling would be helpful in diagnosing your problem.