in this case:
<div apply="org.zkoss.bind.BindComposer"
viewModel="@id('vm') @init('a.b.AVm')">
<div apply="org.zkoss.bind.BindComposer"
viewModel="@id('vm2') @init(vm.vm2)">

the inner vm2 declaration won't work because when EL tried to retrieve 'vm2' from 'vm', vm was not initialized yet.

need to find a better solution for binderComposer vm init.