How to assign multiple agent to activity

Hi,Experts,

i developed a workflow with a activtiy,and set the way to get this activity

is expression,set the agent is general task... but when i run this workflow,

2 user(A and B) will get this activity,and in TCODE swia, it also display these 2 user is succ agent, but when i check the sbwp,only 1 user get the activity..Could anyone give a suggestion? Why only 1 user get the activity?