Add an extra splitter to the inspiratory and expiratory channels of the ventilator, insert one-way valves in the correct directions, and connect inspiratory and expiratory channels directly with a short tube.
The bypass circuit is an optional component: a working setup can also be realized without adding a short circuit that directly connects inspiratory with expiratory channels. In that case one needs to make sure there is at least one patient circuit with a low resistance, meaning at least in one circuit the inspiratory flow is not (heavily) restricted by a flow restrictor or peep valve. In a circuit without bypass circuit, the acceptable limits of flow restriction are dependent on the ventilator type and the flow restrictor used.
