I’m no expert but I think you need to use SectionRoots.
That is how I did the navigation at olson.bz . Basically the wayfinder snippet keeps looking at the parent document until it finds one of the id’s listed in the SectionRoots call.
So in your case it would be a call something along these lines:
[!Wayfinder?&startId=`[[SectionRoots?Sec_roots=7,10]]`&level=`2`&outerTpl=`main_subnavi`&rowTpl=`inner_subnavi`!]
You can find the sectionRoots snippet in the resources. Another option could be to use the UltimateParent snippet.
Hope that helps,
Taff
Adrian Lawley: www.adrianlawley.com
and when you need to go back up another tree use MenuDealer