Two(levers) in the body
Master Chen indicated if we truly understand how to make a lever in the body, then having two of them simply means creating two levers using two different set of body parts.
The 2nd levers must be created separately from the first one. However, they must be related somehow or can be said to share a common point.
Each lever is a gear. Engaging two gears together creates a common point at their teeth, and that is how they relate to each other.
Now consider three gears engaged together. We can rotate only one gear, the two gears will rotate accordingly. Taiji is that we do one action, multiple seemingly separate results are created. Iftwo people are touching the two passive gears, they may feel different directions, but there is only one active gear creating the action, that is the source of power.
Now consider two active gears and one passive gear. If we rotate two gears but in the opposite directions, we can jam the whole system. However, if we can figure a way to coordinate those two gears, it can increase power/speed/freedom of movement for the passive gear. This is like how a 4-cylinder engine work with a timing belt. Each piston action adds something on top of each other, so that they work in a harmonous way.
The real high level of taiji creates a special effect that it breaks the limition of time. The result is something that cannot be achieved in a normal way, like you want to get 5 things through a doorway that is too small for all of them to go through together, and yet they all went through at the same time somehow.
Empty force is created with a lever, which is the attached to the first lever that the opponent is in contact with.