Custom Taxonomy and MultiSites

I have a Custom Taxonomy the contents of which I want to share between a sub-blog and the main-blog. If created on the main blog, the taxonomy works. But if I create a post on a sub-blog, the taxonomy contents are not received by the main blog.

Two questions:

1) How do I store the contents of a custom taxonomy? (code snippet?)
2) With Sitewide Tags, how do I implement the custom taxonomy filter — on the sending or receiving side? (I have the filters on both.)

Thanks much.