Business Unit Conundrum - an account clone in each of two units


We have a CRM 2011 instance which we are about to split into two business units.

For operational reasons, there are a handful of Companies where we would want to have 2 (two!) accounts. One in each business unit, likewise their contacts. Essentially a clone for the new unit. This is the intersection of the accounts and contacts of the two business units. The account managers are precious and want to own their own copy (two lines of business mean two account managers for the same account).

Question, if I have contact as a contact in BOTH business units (i.e. he exists twice), attached to one of two accounts (i.e. the comapny exists twice), and if the visibility of these accounts and contacts is blocked between the business units, what happens if an email is sent to

by a user in Unit 1, by a user in Unit 2

What happens to replies from johndoe to each of these users?

How does CRM assign the activity? Is it perscribed or is it random?

Thanks in advance

Regards MM