I found a mention here:
http://forum.umbraco.org/yaf_postst1600_Multiple-Rich-Text-Editiors-and-Links.aspx but no actual bug report or solution. Yes, you can move your content to another tab but that isn't ideal.
When placing more than one TinyMCE (rich text editor) on a page, the formatting commands only affect the last editor rather than the currently active editor.