aspnet_membership islockedout and session timeout problem

I am using islockedout field of aspnet_membership table for implementing single logon policy. Means if some user is logged on with one user name than other person should not be able to logon with same userid and pwd. So during logon we are checking that if islockedout=1 then do not allow user to login otherwise set islockedout=1.

now problem is that i have also set session timeout=10 minutes in web.config file:
<authentication mode="Forms">
<forms name= ...