1 to 0 or 1 relationship when both entities are optional

In business process which I'm trying to model there are 2 entities which are in 1 to 1 or 0 relationship from both sides, for example: entity_1 might or might not produce one and only one entity_2 record, and entity_2 can be produced without entity_1 participates in the process. So my question is how that case should be modeled? All I found for now is rule that optional entity is child in 1 to 0 or 1 relationships. But what do I do if both are optional? My current assumption that I can make parent either of them, which I think more suitable.