You would want anything in an HA pair at a minimum ideally, or two active active instances like you mentioned accessed via a pool from ltm.
Not sure it will use backplane, thats more for vlan/network visibility between blades physically.
Not sure if b2100 blades can support asm/ltm combo, thats the easiest solution, but from a scalability perspective it makes more sense to have it on separate guests.
I am about to try the asm/ltm/wa combo on some guests soon, but it's on the new 2150 blades with SSD, etc.. I think triple combos require the new blades...
I am also using 2 x 2400 chassis with 3 2150 blades in each chassis, I have HA instances across the two chassis.