A “V” trellis like above allows you to tie your fcanes to either side of the V. This separates them some giving more light… and allows a open space in the center for your new primocanes to come up thru.
Your primocanes come up in the middle… and your fruiting fcanes are on the outside edge for easy fruit picking.
Once your fcanes are finished fruiting… you can cut and remove them… then separate and tie your new primocanes to the outside wires.
Just having support wires in the middle and a single post on each end… can get a bit crazy when you have floricanes fruiting and primocanes coming up in the midst of all that… and only one wire in the middle to tie all that too.
I have done it both ways… but prefer the V trellis now for both blackberries and raspberries.