"Tromix Siamese .223" is the informal name of a double barrel M16 derivative by Tromix Lead Delivery Systems (now Tromix Corporation) of Oklahoma. It is also known as the "Siamese M16" or the "Siamese-16", likening its design to conjoined twins, a.k.a. Siamese twins.
The Tromix rifle was listed on the Tromix Projects on the Tromix official website, one-off experimental creations (by a Tromix affiliate named Šrap) that are not for sale.
The Tromix rifle has the appearance of two joined M16 rifles, one upside-down above the other, joined together by their upper receiver. Their buffer tubes connect by a pad at the rear, forming the stock. Two 5.56 STANAG magazines are loaded into the M16s' magazine wells.
The weapon fires in full-auto, operated by criss-crossing gas pistons. Tubes can be seen crossing between the upper and lower rifle between the hand guards, connecting the two's gas systems. When the trigger is pulled, firing a round from the lower rifle, the gas follows the tube to the upper rifle and cycles the bolt of the top rifle. The top rifle is fired when the bolt closes, and gas from the upper rifle travels back to the lower rifle and cycles the bolt of the lower rifle. 
When firing, the weapon would very quickly alternate fire between the two rifles; the lower rifle fires first, followed by the upper rifle. It would also mean that one trigger pull will fire both rifles once.
Optics can be mounted to the top rifle's handguard at an offset angle, avoiding the rifle's lower receiver which blocks a normal mount. It appears that the weapon cannot be used upside-down - the upper rifle has no trigger.
- ↑ Tromix Siamese M16 Twin Mag Dump - The Firearm Blog
- ↑ 2.0 2.1 2.2 https://web.archive.org/web/20130704112823/https://tromix.com/Projects_o_Tromix.htm
- ↑ 3.0 3.1 3.2 3.3 Tromix Siamese M16: Twin Barrels, Full Auto
- ↑ http://forum.saiga-12.com/index.php?/topic/27283-18-years-of-tromix-prototypesprojects-and-misc-junk-pics/
- ↑ https://www.guns.com/2018/03/30/the-odd-and-forgotten-trimese-16-its-like-an-m16-but-in-a-3-pack-video/