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In two-tier ERP deployments, the operating units operate ERP systems that are different than the ERP system at the corporate level.  This architecture acknowledges the reality that the corporate level is often concerned with financial consolidaton.  The operating units are often concerned with operational issues.  Operations can be quite different from one another.  Some may do discrete manufacturing.  Other operations may do process manufacturing.  Two-tier ERP deployments acknowledge the differences and allow the different business units to stay well-focused and fit to their unique environments.
Keeping each business unit separate is helpful in the event that business units are sold or acquired.  Often, the most important data to share with corporate is the month-end financial results and spend analysis.  Single-tier ERP deployments are convenient for corporate-wide reporting including reports for audit and compliance.   Single-tier deployments make more sense where there is heavier use of shared service centers across a company.  Two-tier ERP deployments are a fit when operations differ widely.   Business units enjoy a bit of autonomy that their own ERP provides.  
A variation is use of the same ERP platform across a corporation but with different ERP instances at different business units.
See also: Two-Tier ERP Deployments Gain Steam. Link: http://bit.ly/lfAwKh.