The Assemble 
and Disassemble 
Multipart Feature tools allow you to combine features into one or take a feature with multiple parts and make them separate features.
To combine features, select the features you want to combine and click Assemble.
To Disassemble, select the features you want to separate and click Disassemble.
The advantage to Disassembling a feature is that now the vertices on the inner polygon can be edited. When assembled only the vertices on the outer polygon can be edited.