What exactly is a 20 amp dedicated line?

My electrician was over this morning installing Porter ports. I had him install 20 amp dedicated lines a while ago. The wall outlets have two places for plugs. It was my impression that a 20 amp line was a separate line for each plug. However, he stated that a 20 amp line was for the double plug instead of the single plug. Luckily I only have one amp plugged into each double outlet. However, If I decided to plug two amps into an outlet with two plugs, wouldn't it negate the 20 amp line? Your comments would be greatly appreciated.