As timill said, recent custom and practice has been the provide one trophy per co-editor/co-author. This was not always the case, I understand, but in the 1950s and 1960s, I take it that the production costs of the Hugo Award trophies were a significant proportion of the total cost of running the Worldcon. Nowadays, those costs are nearly lost in the rounding, representing less than 1% of the cost of running the convention; therefore, we're less reluctant to build a couple of extras.