Instead of explaining the reasons why you might want to use the split table method, you can read this brillant series (written in a Code First perspective, hence the reason for this post):
Associations in EF 4.1 Code First
And to get an example to work from, see this:
How to work with table relationships and entity inheritances in ADO.NET Entity Data Model
How To Do Split Table Model First Style
1. Create your main entity, with a Primary Key eg PersonID - this is the one which will have a corresponding table in the DB eg Person
2. Create your child entity, with a Primary Key - this is the one which will map to certain fields in the DB table but will be a seperate Code entity eg PersonDetails
> Make sure that you name the Primary Key of this entity to be that of the Entity in step 1, eg PersonID
3. Create a 1 to 1 association between the two
4. Right click the association: Properties > Referential Constraint - the Principle will be the entity you created in Step 1 eg Person...note that the Priniciple Key and the Dependent Key should be the same name as defined in Steps 1 and 2
5. Open the Model Browser (right click on some white space in the EDMX Designer > "Model Browser"
6. Right click the Dependent entity (eg PersonDetails) and select 'Table Mapping'
7. In the Mapping Details dialog, select the Physical Principle Table eg Person
> If you don't seen Person here, then renerate the SSDL (store schema) and MSL (mappings) by right clicking on the EDMX white space and choose: "Generate Database from Model"
8. Now map the Table Columns from the Priniciple table to the Scalar Properties of the Dependent entity eg in the 'Column Mappings' bit map "PHOTO: byte" to the Value/Property called "PHOTO : byte"