domain admin access across trust domains

Hi, all
recently, i set up a test environment, there are two domains, in different forest.
one called domain A, the domain admin is adminA
the other is B and domain admin is adminB.

how can I give adminB the same rights as adminA
i mean adminA has join Domain Admins group in Domain A, and i want adminB can join Domain Admins group in Domain A or have the same rights as Domain Admins group A
thanks in advance!