Floating IP the same on HA pair?

I am setting up a pair of BIG-IP VE in my lab and following a HA lab guide from F5.

 

The configuration has me setting different floating IPs for each BIG IP.

 

BIG-IP_1

 

Internal Self IP 10.128.20.240/24 Floating IP 10.128.20.242

 

External Self IP 10.128.10.240/24 Floating IP 10.128.10.242

 

BIG-IP_2

 

Internal Self IP 10.128.20.241/24 Floating IP 10.128.20.243

 

External Self IP 10.128.10.241/24 Floating IP 10.128.10.243

 

The load balanced web page comes up fine with either BIG-IP_1 or BIG-IP_2 as Active.

 

My understanding is that the floating IP should be the same on each BIG-IP. However, when I try to give the Floating IP the same address on both BIG-IPs the web page will only display on BIG-IP_1 and times out if BIG-IP_2 is Active.

 

I am confused by this behavior. Can someone explain?