Depends, there can only be so many Security deckers and Security tally is on a per user basis. So if you deck a host and someone before you has gotten a 10 security tally, you still have 0. The "decoy decker" could distract the Security Decker on "patrol" in the system while the other continues to deck under the radar. However, if the host is on passive or active alert, the security tally is still 0, but you still have to deal with the host in alert-status. Your security tally stays the same when you're still jacked in even if you leave the system. To remove your own security tally you must either jack out completely (by performing a Graceful Logoff operation) and then redeck the system.
Really, that's how I interpreted it, because in SR3 Core, it says that the tally stays the same for everyone, while in Matrix, it says the tally is raised separately. I'd recommend doing whichever makes more sense to you.
Security Deckers are more dangerous because they have Noisy utilities. Since they are usually legitimate icons they can use utilities that are very obvious to the Host and can load better programs with less active memory with the advantage of not-having to worry about security tally. However, if the security decker finds himself invalidated by an Invalidate User operation, he'll quickly find that the IC are much more likely to notice her.