In Japan I observed intensive culture of JMGs where the cane support structures were about 3 feet apart. They will grow into each other so you should plan on spending time bringing the leading tips of the vines back to their home trellis so you won't have a mixed batch of seeds when you harvest them later in the summer. Of course there is a risk of bees making crosses but that is unavoidable unless you tie off some flowers before they open up and tag the resultant seed pod to ensure continuation of that particular strain.

Joseph

Thanks Joseph!

That's really good advice from Joseph. Last summer I had my MGs about that far apart in pots, and by summer's end I couldn't tell which seeds were from which vines in many cases. So I had a problem with that. It was beautiful though, and keeping them in pots allowed me to try new varieties on Nil, Fuji, Purpurea and Tricolor.

Here is a link that might be useful: View of my MGs from one end of fence