You should be able to answer this question better than anyone else. Much depends on what is happening at your restaurant and how it operates. Since you are already doing the job, what happens at night that doesn't happen in the day and vice versa?
How would you divide up what needs to happen each day? Do you arrive early enough each day to get things started? What time is that?
How much time is needed after service at the end of the day to properly close down the kitchen? What time is that?
Those are your starting and ending times. So AM Sous works from that opening for at least eight hours. Closing sous comes in at least eight hours prior to your closing time.
As the only Sous right now, how do your daily activities differ from AM to PM? Those activities would be the ones divided up between the two positions.
And of course, a lot depends on how many staff to be supervised, how many covers you do for lunch or dinner and how much prep is required each day.
Each sous will get a day or two off each week so will one Sous cover the others shift at those times?
It might be helpful to write some of these things down for yourself to help clarify the situation.