Although it's safe to have more than one antispyware program "installed", it's not always good to have more than one antispwyware program "running" and protecting your "real time". (Personally, I don't run ANY of them "real time" but instead using them as stand alone scanners.) Much like antivirus programs, if two "real time" programs are using similar system files, they can have issues. Likewise, if the same infected file is detected by two programs at the same time, glitches can occur as well..
For example, I've had systems lock up which had both Ewido and Webroot Spysweeper running "real time".. Apparently, they both monitor similar things and would cause occasional freezes..
On the other hand, I've tested machines with Ewido and Spybot and in those cases, I've not seen a conflict when they were both running their real-time protection devices.
Hope this helps.