You don't say how tall it is, so there's no indication of how tall "half way up" is. You are going to want it wider at the bottom than the top so I would let them do their own thing for a year . It will be a lot easier to trim off what you don't want than to get width at the bottom if it isn't there.
A pyramid only has a very short leg, so if it were me I wouldn't train it to a cane at all. I bought mine as untrained ones in small pots and I just let them grow as they wanted for a couple of years before starting clipping - they are now nice little pyramids about three feet tall.Maybe someone else will disagree. They are slow growers so you can't hurry them.

I agree with Steragram - let them do their own thing for a while and you don't need a long stem. If you imagine them in the pot - you don't see the stem and the foliage is very close to the top edge of the pot.
They are very forgiving once they get going so if you make a mistake in shaping them it doesn't take long for it to grow out. I speak from experience ;)

Thank you both. I will do as advised and let them get on with it without my interference.