Recent versions of Windows Defender in Win7/Win8, being built into Windows, are explicitly compatible (thanks to changes made by other antivirus vendors, as well as changes made by Microsoft themselves) with most third-party virus scanners. That is, you won't see BSODs, etc. if you are using the latest version of Windows Defender combined with the latest version of any self-respecting antivirus program from a third party.
Your question was is it okay. Yes, it's okay. Your PC won't crash. It may run slower; it may run much slower -- but it'll work.
You will notice that, if you use certain websites that run your upload through 40+ virus scanners, almost every executable you can find will flag something in at least one virus scanner. On the other hand, actual viruses that are malicious will only be detected by some of the virus scanners, while not others.
If you were super paranoid, you could install numerous virus scanners and let them all do "on-access scanning" at once. This would drive CPU through the roof when doing writes to disk, and you would be getting loads of false positive results, but it'd work. Remember, although you marginally increase your chances of finding actual malicious programs by running more virus scanners, you also dramatically increase your chances of false positives. So you'll constantly be clicking on "shut up, it's okay!" dialog boxes and pop-ups. With 40 virus scanners it would be completely unmanageable.
However, TWO virus scanners is not unheard of. For example, you might run Windows Defender and Malwarebytes, or Norton and Kaspersky, or AVG and McAfee. It depends on which products you're invested in, which products you trust and which you don't, and how much you research on the internet to determine the reliability and business practices of each company, when you are selecting virus scanners.
The long story short is that currently, in 2014, the latest version of Windows Defender plus the latest version of "top tier" virus scanners (that is, ones which are widely regarded as actually good) do not conflict -- there are no compatibility issues whatsoever. This advice applies primarily to Windows 7 and later; I can't speak for the older OSes. But you will notice performance degradation if you run more than two virus scanners. For two itself, I don't think you'd really notice. I personally run Windows Defender and Malwarebytes.
Also note that some virus scanners may choose, as part of their installation program, to disable Windows Defender for you, assuming that their virus scanner is the only one you could possibly want. In that case, they are compatible, but you're only benefiting from the virus-scanning abilities of one scanner, not two.