Plan "1" split and half of the participants were merged into a mirror Plan "2" effective 9/1/07. Plan 2 has an inst'l parent company that has a differnt plan with a differnt vendor. What does Plan 2 need to do regarding testing from 09/01/07-12/31/07? Do they test as a sep identity for that time period? test with Plan 1, with the parent co.....?

I'm confused (a normal circumstance these days). Who owns what?

See IRC §410(b)(6)© and Treas Reg §1.410(b)-2(f).