Two prefix commands for tmux?

The above answer is correct for tmux 1.5 and earlier

However, this has changed in tmux 1.6. Now, there are two options, prefix, and prefix2, each of which can only be set to a single value, like this:

set-option -g prefix C-a
set-option -g prefix2 C-b

For releases prior to 2012: Issue the command only once, with all the desired prefix keys separated by commas:

set-option -g prefix C-a,C-b

For versions >= 1.6 (Released the day before your question, 23/01/2012) see the other answer.