Basically, it's my personal system (running 10.10) where I often switch from myself to my other myself (one login is for personal purposes, and the other one is the working login). So I come to situation that both myself are logged onto the system because I want to switch between accounts quickly.
Does anyone know how much memory has been consumed in such work? I haven't noticed any slowdowns, but I would like to know that. Is there any Ubuntu official document describing this?
There can't be really be any documentation for this as the memory usage depends on the running application (both open and those in background) and on your system (for example, desktop effects increase memory usage).
You'll find the memory increase by opening the system manager when you have one user logged in and then when you have two users logged in. Don't open any application when you do the test, that way other programs won't affect your test result.
The memory usage should be pretty close to if you were running a single user, so long as you are not running applications that use a lot of memory, and in either case, if you begin to run low on memory the system will swap some out as Gilles mentioned.
If you are curious to know your memory usage, just run the system-monitor:
More than if there was a single user, but less than twice as much. It's extremely hard to be more specific, because it depends a lot on the applications.
When you switch to a different user, most of the applications of the idle users aren't going to be doing anything. If the system needs more memory for the active applications, the idle applications will be swapped out. That's likely to mean the applications of the idle users, but Linux is pretty good at deciding what to swap out, and the fact that there are different users won't enter into consideration.