twin needle domestic sewing machines

[chrisash]

I notice my wife's domestic Janome machine can use a twin needle adaptor, but only has a single bobbin

This intrigues me, so can someone explain how the single bobbin works with two needle, thinking about it how does any zig zag bobbin work as the needle is never in the same position

[Michiel]

The hook catches both  threads

on the top side you see two rows of an straight stitch

But on the under side you get something that looks like an strange zigzag

[Michiel]

That is why on an zigzag machine the hook is orientated the way it is (on the backside) :

it needs to catch the topthread in both needle positions (left and right)