state_machine.c using 2 bio_s_mem's

[Yannay Alon-BAY004]

Hello all
 
In demos/state_machine/state_machine.c two bio_s_mem's are used as read
and write bio's under SSL.
 
What are major differences between two bio_s_mem's versus one bio pair
(2 bio_s_bio's)?
Why would i prefer one over the other?
 
Best Wishes
Alon