Depending on how you wish to implement it, this might be achievable.

When you create a Subdashboard in the 'Home' dashboard these will
automatically be displayed on the top level with the document library,
search dashboards etc
However if you were to create subdashboards in any other dashboard say
dashboard x, then once you open dashboard x your sub dashboards will be
listed beneath it on a separate line.

This came up on the list some months ago. I believe the answer was no, you
can't split the bar  onto two lines  without massive custom work.

Also, it's unfortunate that you can't have more than two rows of nav bars
either.

If you want to pursue this you can look to customising  dbview_ie.xls in
the portal/resources folder. This is the xsl stylesheet that ultimately
transforms the dashboard definition into HTML and is where the
navigation bar  is generated.

One trick I have used is to hide dashboards from the navigation  bar if
they have a '1' in their partstorage node. Then you can design your own
navigation in a Web Part.