That is one of my big questions about Boring. How will they connect equal sized branch tunnels?Could a TBM successfully tunnel through another tunnel at right angles? if so, this techniques could be used to easily create living and storage space on Mars.
The movement of the head requires a solid ring behind it, so any intersecting tunnel blocks the previous. Additionally, the outer ring grout would not be contained.
Specially designed crossing pieces (cutting the cylinder kills the strength) that get removed or cut out afterward and a way to seal off the grout layer might work.