you can use 2 separate screens. i have done that before just make 2 screens that will fit your space and fasten 1 on each side and keep 1 strain under 1 screen and the second strain nder the other screen. that way you can harvest separately. i personally don't even like to flower 2 seed plants of the same strain under the same screen. you can have different phenos of the same strain that grow very differently. i always only used clones from the same mother for single screen scrogs.