That's as in "Two _________, no waiting." Fill in the blanks with a particular instrument. These are bands that have two people in them that play the same instument. Most of the time they're both dedicated to the instrument but there are a few multi-instrumentalists (who always seem to find their way into prog bands) who do the doubling up. See the setlist at
http://fusionreactor.blogspot.com when I get around to putting it up there.