The official novelisation deals with this quite neatly. The fundamental basis of The Breach is that it's a fold in spacetime, bringing two distant points (in this case separated dimensionally as well as spatially) into contact. That being the case, you could no more create a "one way breach" than you could poke a "one-way hole" in a piece of paper.
Harnessing the fundamental energies necessary to the creation of a
passage such as the Breach — which essentially folds space-time around
itself to bring two distant points into proximity — requires
technology far beyond current human capabilities, as well as focused
energies equivalent to the entire output of human civilization during
the last century. Destroying the Breach, however, is likely easier
than creating one.
Secondly, a uni-directional passage would defeat the aliens's battle plan; to learn from each Kaiju encounter and use that data to refine their attacks:
- The kaiju are manufactured
- Some of the repeated strands of DNA act as encoding mechanisms for a kind of species memory
- The kaiju passing through the Breach transmit their experiences on Earth back to the Anteverse