Entity Framework has no support for triggers, although you can certainly manually execute a statement that would create a trigger, but you would need to do this after the table was created (if using migrations).
You can use the technique specified by Ladislav in EF 4.1 code-first adding a trigger to a table
Take note of his warning, however, EF will not be aware of any changes made in the trigger. If your intent is merely to ensure that 2 columns in a table are not null, you’d be better served with a constraint (constraints are also not supported by EF, but you can add them manually).