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