When a unit is under neural parasite, it temporarily does not belong to you. When your own mothership is under neural parasite, can you start training a new one? When the new one completes, you will end up with two motherships. Does this work?
Units under the effect of neural parasite are simply given a "debuff" to indicate control is temporarily transferred. The unit is still "yours", which means you still have to pay for the supply cost of the unit as well. The mothership restriction is also in play.
Try this out in a unit tester to visualize this scenario better.