LACK Attack #1.5: Double Wide

I love the LACK Attack series of mods. But they deal only with growing the lack stack in the Z axis. For my project I use double-wide setup, so I designed new type of joint, which can join two LACKs side by side.

The design is inspired by the original [LACK Attack #1](https://www.thingiverse.com/thing:2586887) adapters and is intended to be used with them.

* `LackJoint_A.stl` is intended to be used on top of the LACKs (to prevent the tables from coming apart) or at the bottom of the entire stack. There are two screw holes to attach the adapters to legs to prevent them falling down, but in my case they sit snugly and there is no need to screw them.
* `LackJoint_B.stl` is to be used on legs of the second (or higher) row of tables. It will hold the legs together and also has the "lip" seen on original model to help stability.