This structure is no longer functional as Patch 2.3 makes it impossible to build bunkers close to each other, in the form of an error message.
Ladies and gentlemen, boy and girls, May we present.... The Invincible bunker! (also known as double bunker, indestructible bunker, etc.) Yes, that's right, for just 10TP you can get a bunker that gives double, yes double the protection, it gives no hitbox overlap and double Protection overlap. As an added bonus, you can build one of these on the top half, stuff 6 troops in it and laugh as Wespes pound futily upon its concrete invincibility.
Credit to Suckyjack for R&D
The Theory Behind It
On the left you shall see a bunker with marked hit-box and protection area. In the early days of Mud and Blood 2, many strategists stacked bunkers on top of each other. However, since they were stacked fairly close, the hitboxes overlapped, causing a lucky arty shell to wipe both bunkers, and 10 TP, out. In addition, these primitive double-bunker systems also had LoS issues which cut the effectivity of the gunner inside. However, SuckyJack saw this and improved upon it, creating a bunker with no hitbox overlap while providing double protection. Therefore, any units at the top center of these structures will get hit with impunity. However, it does not guarantee effectivity on direct-fire units, such as PAK-38's. The structure is much more effective against Indirect Fire, such as Wespes and artillery, where fire will be directed to the unit, not the bunker.