How long the branches take to grow depends on how healthy you can keep it. I have a chinese juniper that started some fresh branches two years ago. Right now they're moving towards secondary and tertiary branching, in two more years they should be long enough to be wired decently.
For now, I would go for the double trunk. And I would plan on removing the branch moving to the left in the second picture, because it crosses other branches. But! Instead of growing entirely new branches from the start, you could use the branch you remove as a donor; take the finer branches off and graft them back on to your trunk.
I made some instructions on grafting, with pictures. I know some people love the ziplock bags, I love parafilm because it breathes.
What you'll need:
- 25cm of parafilm per graft (if they are as small as mine, it's always better to have more parafilm!)
- A super sharp knife with a straight edge.
- A piece of wood.
- A scion (branch to graft onto another).
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Pre-stretch the parafilm. This is going to take a few tries, it's difficult material to begin with, over time it gets easier. That's why it's better to have more.
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Wrap the scion loosely in parafilm, make sure it closes all over. Wrap halfway over the previous winding. If you leave open spaces, just use another piece of parafilm to patch the hole; it will stick to itself when you stretch it.
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I gently scraped off the old scale foliage. Then make a nice clean long cut on one side.
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Make a significantly smaller cut on the other side.
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Here's a side view.
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Make one clean cut in the branch, down to the 'white' wood. Insert the scion so that the 'long' side of your cut is facing towards the trunk or branch. This cut was a little on the big side. I feel fine making and documenting mistakes.
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Wrap heavily with parafilm, so that it closes the cut on the branch with the scion pressed between it. If needed, because of the wind or other weather, add a layer of wire (I prefer aluminium wire because it's softer).
I did not add wire in the picture below, because this juniper is sheltered and the bushy exterior foliage catches all the wind. Also, just because I was being lazy.
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When you wrap with parafilm in the second to last step, make sure you don't move the scion. Some people use needles to keep it in place, and only pull on the parafilm when it's on the back side of the receiving branch. If you pull on the front side, where your scion is, it's going to move out of place and you can't see it through the parafilm. The scion will die in a month or two, and only when you unwrap it, it's going to show you what happened.
In my back yard, the scions start growing through the parafilm after two and a half months. I don't touch them in the mean time. When they show good growth, I cut the parafilm on the scion at the sides and let it alone for another two or three weeks so it can breathe. I leave the parafilm on the trunk until winter, it comes off easily when it's colder and I don't want to mess up the tissue that's healing.
The grafts in my example are very likely to not hold. I did a crappy job and it's too early in the year. But I wanted to practice a little.