In the first part of this series I had an overview of
Exchange migration which can be found here.

In the second part of this series I provided details on how
to check for inconsistencies on user attributes and set for UPN which can be
found here.

In this part of the series I will give you details on how to
setup the coexistence. So here are the steps to configure coexistence:

• 
  Conditional
  Forwarding: As you have 2 different forests you will need to have DNS name
  resolution between the domains. You can use DNS conditional forwarding feature
  to do this.
• 
  Trust
  relationship: some of the tools that would be needed for migration (Hint
  ADMT) will need Windows trusts to be configured between the 2 forests. You will
  also need to configure Windows trusts for cross-forest availability.
• 
  Directory
  synchronization: After you start migrating the users, you need to make sure
  users are available on both sides. The recommended approach is to use FIM to
  synchronize users, distribution groups and contacts. While you are configuring
  you need to plan for migrating the users through your migration planning and
  will need to configure the new object provisioning through FIM. (Ex: What will
  happen when a new user is created in old forest during coexistence)
• 
  Control
  panel: remember our scenario. We are moving only Exchange functionality to
  the new forest. In this case you might consider using control panel to manage
  Exchange properties of the users. If this is the case you might have to do
  configuration on your Control Panel.
• 
  Coexistence
  Server: In order to migrate users and provide mail flow you can use an
  Exchange Server 2010 in the old forest. This will provide you with the new
  mailbox replication proxy functionality. You would also use this server
  together with the Exchange in new forest for providing availability services in
  cross-forest migration scenario. You would need to have certificate installed
  on this server that would be trusted by the new forest Exchange servers.
• 
  E-mail
  address policies: In order to flow mail between two organizations you will
  need to configure secondary e-mail addresses for each side.
• 
  Send and
  receive connectors: These will be needed on both sides to enable mail flow
  between 2 Exchange organizations acting as a single organization.
• 
  Cross-forest
  Availability: During mailbox migration you may want to have each side of
  your Exchange servers to be able to query availability information for
  respective recipients. For more information have a look here.
• 
  Auto
  discovery: You will need to configure auto discovery services so that one
  you start migrating the users, they will be able to reconfigure themselves for
  the new forest. Please keep in mind that this will work for seamlessly for
  Outlook anywhere and ActiveSync but if you configure the coexistence server as
  your Internet facing CAS servers you will only get a redirect, which means
  migrated users will be prompted for authentication on new servers. You can use
  an Access gateway solution to provide seamless redirection when the mailbox has
  been moved and they are accessing through OWA.

These are the basic steps you will need to do configure
Coexistence between 2 forests.