Two Forests/Domains in to One

Our company has two separate domain/forests at 2 different locations connected by a VPN WAN and external trust. Both PDCs are Server2K8R2.
One named A.CompanyName.Internal, the other B.CompanyName.Internal, again both separate domains in separate forests with a external trust.
We want to consolidate the domains so that we can invoke Domain wide DFS for file replication and centralized backups.
What would be the best way to do this? A domain migration from B.CompanyName.Internal to A.CompanyName.Internal is one possibility (a lot of work). Is there an easier faster method? Parent/Child Domain?
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First decide which domain you are keeping and which one will be eliminated
Document all user rights and groups.
Migrate the users first.
Then dcpromo  the serer to be removed.
Then dcpromo it into the new domain.

Make sure that you have good backups. You should test this in a test environment(something like vmware).

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