A room with two adjacent openings:
A. An opening behind the door:
If behind the door there is another opening [such as to its right or left], then if it has a support beam of one Tefach wide [8 cm], then both openings are obligated to have a Mezuzah. [See pic]
B. Two doors that are separated by a pillar which enter to the same room:
If one has an opening with two doors, then if there is a pillar in between the two doors [of one Tefach] and the hinges of the doors are attached to the middle doorpost, then they are considered two separate entrances and both require a Mezuzah. However if the doors are attached to hinges on the side doorposts, [and both doors open together] then only one Mezuzah is required, as the middle doorpost is only there for beauty.
C. Two door-less openings by corner:
A room opened partially on its west and north side, thus forming an opened corner area that has a pillar standing by its corner, only requires one Mezuzah if the pillar is less than a Tefach. However if it is a Tefach then it requires a Mezuzah on both its west and north entrance.
 Michaber 286/20
 Michaber 286/21
 If the pillar is less than a Tefach wide then it never requires two mezuzahs, one by each door. [Pischeiy Shearim 286 as ruled in Michaber 286/20]
 Taz 286/12; However if one door remains closed while the other is opened then they need two Mezuzahs. [ibid]
 If one door is attached to the pillar, while the other to the side doorpost one is to place a Mezuzah by both sides but with only one blessing. [Pischeiy Shearim 286]
 Shach 286/27