There's a main forest (company.local), and two company forests (companyA.company.local, companyB.company.local). I've read the post here about making the people picker span several forests, but haven't implemented it yet.
I'm curious how this works... On one site collection when granting permissions, the people picker can find companyA\userA, but no one in companyB. On a separate site collection it's the reverse. What determines which forest a site collection's people picker will search? Is it based on the Site Collections Primary Administrator?
We have User Profile Sync importing from both forests, but I'm guessing that UPS really doesn't have any effect on people picker and user permissions.