The European Space Agency is currently using two-armed 3D printing.
NASA's counterpart across the pond is using twin robotic arms that work together to 3D-print and mill a test version of the optical heart that could be used in the Athena X-ray observatory.
When it's done, it will be the largest and most complex object ever 3D-printed in titanium — and if you can dispute that, email me.
The arms are currently making a test version of an optic bench about 3 meters in diameter. The first robot uses DMLS technology to build by melting titanium powder. The second robotic arm then immediately cuts away any imperfections using a cryogenically cooled milling tool. The bench itself is placed on a slowly moving 3.4-meter diameter turntable.
ESA is working with the Fraunhofer Institute for Material, and Athena's final optic bench design is still up for debate.
Scheduled for launch in 2031, the Athena mission will probe up to 100 times deeper into the cosmos than previous attempts. In order to make that happen, the optic bench will align about 750 mirror modules — each of which is formed of 140 industrial silicon mirror plates, stacked together by a sophisticated robotic system — and the smallest imperfection could spell disaster.