There is a couple of ways you could go about this. You could choose the manual approach by exporting both lists to a single TSV file and split it manually in Excel, then import them each as a separate list. Or an easier way would be to, on the first broadcast, select both lists in the distribution settings area, click on the advanced tab and add "limit XX" (XX being the number of your total subscribers divided by two) to the end.

Once the first email has finished broadcasting you can send the second email to those who didn't receive the first by using the 'basic' section, selecting both lists again, then click "Refine" and select "not sent any of activity" followed by the activity you just broadcast the first time. Remember to use the "Run Check" feature next to the proof buttons in order to check the number of subscribers each expression will target.