Helping a plumeria to branch.

I have two plumerias, and as you can see from my location, am not in an ideal place for these plants.
There are each more than six or seven years old. The oldest (which I got at a plant show as a "stick") has bloomed, and has also done a little branching, but not much lately. The other, has neither bloomed nor branched.
I have tried cutting back the newer plant a number of times, but each time all it does is send out a single branch below the cut. Thus it is a series of single branches reaching to the sky.
Is there anything I can do, other than moving to a tropical climate?
Jerry Bank
Trenton, New Jersey
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