Can I define one list filter, and connect it to two lists on one web part page? I have two seperate lists on a page, both containing the 'author' column. Now, I want to add a filter to de page, so you can select one specific author, and so that both lists only show items from that author. Is that possible in SP 2010?
Yes this can be done. Look into Filter Web Parts and connecting the filter to your list web parts on the web part page.
An example is to add a filter web part to the page and then connect it to the two list web parts on that page. There are a couple of filter web parts so you may need to play to see which one fits your needs best.
Here is information on the Filter Web Parts: http://office.microsoft.com/en-us/sharepoint-foundation-help/work-with-filter-web-parts-HA101794720.aspx
Here is a basic article on connecting the web parts: http://www.mssharepointtips.com/tip.asp?id=1034