Yes, in special cases. Not a bad idea. It totally depends on the set of features you want to use and the tolerance of your design to the difference between browsers.
Besides, in some cases putting several properties in the same CSS is a valid technique to write a cross-browser style. For example, consider a popular feature: rounded corners. Look at this article:
http://www.css3.info/preview/rounded-border/[
^], pat attention to the section "Cross Browser Examples". You can see that the same property has different name is different HTML layout engine. The technique is using them all. The matching property name is used by appropriate browser, all others are ignored.
I personally prefer a single CSS trough strict simplistic design which makes the difference between browser non-critical. Such simplistic design stands out pretty well compared to over-decorated and insufficiently stable design and layout of Web sites dominating the Web these days. Remember: Web site is valued for its functionality, ease of access and content. However, I don't want to delve too deep into a very different field — industrial design. You can find your own style and establish your own palette of expressive tools.
Good luck,
—SA
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