Wallsharing

A five by five by five cube of silver riveted blocks, with four large fan housings on the nearest side. The right side has two controllers barely visible. — Quad Steam Turbine

Wallsharing is a behavior of virtually all GT multiblocks that allows them to have one or more walls in common with another multiblock of the same or even a different type. This can be used to parallelize production lines, economize on space or save construction resources.

To wallshare, two or more machine controllers need to be placed in such a way that all the shared blocks are in legal locations for all conjoined multiblocks. When multis cannot wallshare it's due to their inherent design which lacks the necessary symmetry. Wallsharing is not a unique feature that needs to be explicitly enabled; it will always work where physically possible as a controller only cares about the blocks within its defined limits, not what's adjacent or built nearby. As long as the structure of each multi individually is correct, it will continue working as if nothing else around it existed.

Limitations

Most multiblocks can be mirrored or rotated to facilitate wallsharing. Exceptions include the Charcoal Pile Igniter, Bricked Blast Furnace, Distillation Tower, Ore Drilling Plants and Void Miners, Industrial Washing Plant, Algae Farm, Bacterial Vat, Deep Earth Heating Pump and Mega Blast Furnace, which only have horizontal wallsharing potential.

Care should be taken when choosing which blocks to wallshare. Energy Hatches are very difficult to share and at most can be used by two controllers since each only accepts 2A. Doing so with recipes that take close to or exactly 2A is risky as a single Maintenance issue will cause the sharing multis to void and shut down. It's much safer to share Energy Hatches when the total recipe cost is 1A or less, since that affords some slack to address any repairs without loss. Standard Maintenance Hatches should not be wallshared; they can but will only repair one multiblock, so each Controller needs its own. A LuV tier Auto Maintenance Hatch can be shared between multiblocks and will use items to repair any it's a part of. The UV tier Auto-Taping Maintenance Hatch cannot be shared. Mufflers can be shared, potentially even with mirrored multis provided that the Muffler has an air block in front of it.

Multiblock machines from other mods cannot wallshare. Many will not even form with another multi of their type adjacent, such as Coke Ovens and Water Tanks.