No, like all antivirus you can not have two real-time antivirus operating together. Since there's virtually no way to truly disable such suites, only one complete suite (AV/AS/Firewall) can be installed at one time. This isn't nasty, it's common sense.
However, you can have some other antispyware and even antimalware installed as long as you don't operate the real-time components of that application. In some cases it might even be possible to use the real-time components, but it wouldn't be recommended since even if there aren't conflicts, the multiple detections and notifications that are likely to result would be confusing.
Since I am a moderator on the Windows Live OneCare forums and also an Advisor Group member here, I'm fully aware of the realities and confusion surrounding such situations. I have personally recommended only one active antivirus/antispyware for over a year, well before I ever became involved in testing OneCare earlier this year and then a moderator in June.
Misunderstanding like yours is exactly the reason that even the freeware products have stopped recommending many products be combined, since new real-time abilities have created conflicts and the average person can't understand the potential interactions between them. Even someone with programming background like myself can find these relationships confusing and not worth the effort.
As long as you avoid enabling multiple real-time components though, you can use additional scanners like Spybot S&D with suites like OneCare. Just be prepared for occasional false positives that may occur when one product mis-identifies components of the other as malware, since this is a normal side effect of such combinations.
As always, you're free to use which ever [combination of] products you prefer. Just realize that you then take responsibility for the potential interactions between these products, since no organization can guarantee they won't happen, and would in fact be fools if they did.
Bitman (OneCareBear at WLOC forums)