Discussion Groups / English Usage / January 2004

"I can do two things at a time" means that I can do both things at the SAME time.  

"At one time", I was able to do two things at a time.  This means "Some time ago, I was able to do two things at a time."  (It implies that I can no longer do this.)
 What is the difference between this two phrases ? they are confusing somehow

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 www.englishdaily626.com

    It could be argued that "at a time" is the only right expression,
since any point in time can only be one point. Imagine a line with
measurements for hours, minutes, seconds...right down to nanoseconds
if that helps you. Now imagine that there is an event that takes place
at a point along that line. Whatever point it is, it is ONE point, so
there is no point using the phrase "at one time", since the word "one"
in there is redundant. There are times though when "at one time"
sounds appropriate. If you are say, in your fifties, and are lecturing
a group of people in their 20s about how things were when you were
their age, it would sound better to say "At one time, when a man was
20 years old..." than it would to say "at a time". It's probably
acceptable to use the phrase "at one time", as long as it sounds okay
in the context of what's said or written. It's probably not okay to
write a response this long without breaking it into at least two
paragraphs, however.