lintl size for door opening in cavity wall

Hi,

I want to make a 30" wide door opening in a cavity wall in my hallway. The wall is the gable wall of my 3 bed semi d house. The first and second floor joists rest on this wall. The doorway will allow access from hallway to existing extension on other side of wall (so will be a standard internal door).

So, what size lintl do i require?
Will it be two lintl's (one for each block leaf).
Does the opening need to be acropropped given its only 30" or so wide

thanks in advance,

Brian