Hi Berting,
I have seen reports of people using multiple site rating services without any problems. SiteAdvisor simply checks a database for the latest McAfee ratings of sites and displays the results. However, it is probably not really necessary to use more than one service, since most will be in agreement most of the time, especially regarding sites that are well-known for hosting malicious content. These days, most of the online threats come from mailicious ads or scripts injected into "Safe" sites. Since the sites, having been tested at some point in the past will show as trusted, and the recently installed malware will usually only be present for a short time, these site rating services are really not that useful for indicating what legitimate sites may currently be compromised. One rating service should certainly be sufficient for warning you away from known bad sites. The more practical danger posed by good sites that have been hacked or seeded with malvertisements will not be lessened for you, no matter how many rating services you use.
In fact I do have McAfee site advisor installed on my computer, along with NIS 2011. Here is the screenshot, Googling Symantec.
