Just to corect my original post, it turns out that Arial Unicode MS does have the multiple forms needed to support both Chinese and Japanese. However, you'd need to be working with an app that supports the 'locl' OpenType feature.

In another comment: The closer you can get to a universal font is the DejaVu font project.

There are a number of other fonts with vastly greater Unicode coverage than DejaVu. As noted, it is missing Chinese, Japanese, and Korean. As the original poster was specifically looking for Chinese and Japanese, it is not a reasonable recommendation.

That being said, for Latin and Latin-related writing systems, the DejaVu fonts have pretty good coverage.

Cheers,

T