EDIT: Let me just preface this by saying that 'barriers' connecting 'natures' is to vague for a meaningful answer as your question ultimately depends on the properties of the 'natures' and the 'barriers'.
EDIT 2: Don't call your idea a 'principal', people will be (slightly) less likely to take you seriously.
The examples you provided are good, however, I trust that they are examples of manufactured containment or control of the passageway.
Allot of stuff both natural and unnatural is one way or favors one way like Danger has said... even in the natural subatomic particles themselves. The process of time itself. We can give counter-examples all day but let's try something else...
Your trying to draw a conclusion that if you went from one 'nature' to another 'nature' then someone from the other 'nature' should be able to cross over to your 'nature' as easily as you went into theirs... I don't see any reason this should be.
Also, your examples are flawed.
A needle pierces through the skin and injects a solution into the body. As the needle is removed from the skin, the puncture continues to exist (the opening or passage) and allows for blood to exit from the body.
The pressure you put on the plunger makes it much easier to leave the needle to go back inside... this is really an example of a one-way portal.
Pilgrims get on a boat and travel across the ocean from one continent to a second continent. After the pilgrims enter into the second continent the natives of the second continent use the boat to travel back across the ocean and enter into the first continent.
Not if the ship was disassembled, or crashed on shore... In which case it would be more of a one way portal.
An astronaut flies a spacecraft onto a distant planet. He enters onto the planet. A microorganism from the planet enters into the spacecraft and returns to the astronaut's home planet.
Not if the ship is out of fuel... then it would be a perfect one way portal.
So, essentially, once a barrier, separating two natures, is opened, then, regardless of which nature enters first, theoretically, the second nature has an equal opportunity to access the passage and enter into the first.
If you drew that conclusion from your examples (or similar) I would reevaluate your claim.
I'm interested in learning the physic's principle to the following query. It's not homework. Just wanting to learn what I'm presuming is a fundamental in physics.
Next time, start with a physics principal and then draw conclusions from that. It's when you start trying to go the other way that you start turning in Ramtha.