Answer
Rather than trying to make the shared-login collision notification more reliable, the real fix is to stop sharing a single login among multiple users — purchase individual LTU (Limited Task User) licenses for each person instead. IFS's licensing and session model isn't built around multiple people sharing one login concurrently, which is exactly why the collision-handling behavior (sometimes warning, sometimes silently kicking the other session) is inconsistent; separate named LTU accounts for each user avoids the problem at its source rather than trying to work around unreliable collision-notification behavior.