In general, I feel that practicing with scales is a good way to learn technique like this, be it crescendoing through a passage, playing at different dynamics, playing with different rhythms, etc.
For instance, play an octave of a scale with your right hand loudly. Then play an octave of the scale with your left hand, but softly. Then practice playing both hands together at a slow tempo. As you are able to do this without thinking so hard about it, increase the tempo. It is important to learn to be able to do this without concentrating on the dynamic with which you play each and every finger.
Once you can do this, extract the method out to the passage you are playing with. Theoretically, it will become easier for you to move directly towards practicing with both hands together at a slow tempo and increasing.
This is also the method I use for learning to play different rhythms with each hand.