How much to cut back loganberries

I planted a loganberry last year and this year it's grown on very well. I've got it in a bed alongside a wall, with horizontal wires on the wall to tie it in. Looking at it now, I want to tidy it up a bit for autumn and tie it into the wires, but I'm not quite sure what to do with it. It seems to have just put off two extremely vigorous shoots, both of which are way higher than the top of the wall, and left to themselves they've just shot up, doubled back and gone back down to the ground again. Should I cut these back to the top of the wall or should I just let them do their own thing?

Thanks. The shoots have to be 15 to 20 feet long so I had no idea what to do with them. Even wild brambles don't put off shoots as long as that