Generically speaking the average angle of elevation will be 90 degrees - your latitude. But there is a +/- 23 degree variation over the
course of the year.
So if your latitude is 35 degrees north, then the angle of the sun will vary between a low of 32 degrees on the winter solstice up to 78 degrees on the summer solstice.
To make sure the southward wall doesn't shade out the northern wall, you'd have to do a little trigonometry. Take the height of the south wall, h, and apply the tangent.
To keep the north wall shade free all year long, we would use the winter solstice angle.
Distance between walls > h / Tan( 32 )
You can adjust this formula to different angles if you are only concerned about sun from during the warm parts of the year by using higher angles. Equinox to equinox would use an angle of 55 degrees in this example.
Hope that helps!