I transplanted mine (using netted coir) into a larger (2.5 inch square this year) pot, let them grow until the both have three or four sets of true leaves and are fairly root-bound (this is the key).
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Then I water them, not soaked, but moist all the way through, de-pot them, and, using a sharp kitchen knife, carefully saw in between the two stems through the rootball. This should result in two equal rootballs which I then put into larger pots.
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Out of about sixty 'doubles' I had this year, I only lost one because I was not careful and actully cut the stem of one at soil level.