Hey Carrie,
There are a couple of ways to do that. It really depends on how you want to build the Program, if you want it to scale/send over time, or if it's a one-time (and so on).
One way would be to create a campaign that uses Random Sample to split your contacts in half (adding them to static lists). Then, you create scheduled campaigns to send your email to either list whenever you want (such as 10AM Tuesday for List A, 7AM Sunday for List B). And you don't have to use Random Sample; you could use whatever qualifying criteria you want and have Marketo send leads to lists based on those for your send.