You "can't" do that. Well, you can, but you risk destroying your voltage sources as current will flow from the higher voltage into the lower voltage and many voltage sources are not meant to sink current.
The standard way of using multiple power supplies on one line is decoupling via diodes or equivalently controlled switches (typically MOSFETs) acting as controlled "ideal diodes".
A useful description is given e.g. in
this datasheet.