Sugar Community Edition 6.0 Documentation:

User Session Rules
For security purposes, Sugar has set the following rules for user sessions:
- You cannot log into a Sugar instance simultaneously from two different machines
or browsers.
I can log into system simultaneously from two different machines
and with several browsers from one machine (FF+IE+ Chrome)

Sugar CE 6.0 on
Linux Slackware 10
Apache 2.2.13
Mysql 5.0.24
PHP 5.2.13