Keep your Activity Monitor open, and watch in the CPU, and Memory tabs. The Energy tab can show good info, too.
You are looking for processes that are at, or near the top for CPU, etc.
How long ago did you update? If it has been less than a couple of days, then you should simply allow that to happen, as some updates may reset, or trigger system optimization, or just need to run some of the processes, such as those in the spotlight group of processes. Many of those have the letters MDS in the name somewhere.
You might find that letting those processes run for another day or two may make a big difference in battery life (because the CPU and other energy use should diminish after allowing some optimizing time.
I suspect that all things being equal, that having two accounts logged in may increase the energy load (and decrease battery life as one result) depending on what you have the two accounts doing...
Log out, rather than having some kind of task running in the background...
Did you start the multiple logins AFTER you updated the system -- or is that something that you have always done?
