Concrete lintel for window in blockwork wall

[tvrulesme]

I am looking to install a small (500mm wide x 700mm high) window in an existing garage wall. 

 

The wall is a solid wall mix of brick and block (left hand side in this picture)

 

 

As the wall is appox 215mm thick I was thinking of installing 2 x pre-stressed reinforced concrete lintel 900mm x 100mm side by side before cutting the 500mm wide opening for the window itself.

 

The building is single story and has a lightweight roof covered in felt. 

 

Does this sound sufficient or overkill? Should I just put a single lintel facing the outside and fill the inside with brickwork?

 

Same wall from the outside showing the position of the window

 

 

 

[markc]

A couple of lintels are overkill loading wise but as they cost very little and are convenient they are the way to go