I know its not an approved method but when I use a larger pole like the OP had in the picture, I cut off a straight piece of the smaller mast about a foot long, stick it into the larger mast, and run a minimum of two large bolts with nuts completely through the entire thing perpendicular to each other using a drill bit first for the hole. I then tighten both until the larger mast is basically crimped against the smaller one on all 4 sides. Again, not approved but extremely sturdy.
I know its not an approved method but when I use a larger pole like the OP had in the picture, I cut off a straight piece of the smaller mast about a foot long, stick it into the larger mast, and run a minimum of two large bolts with nuts completely through the entire thing perpendicular to each other using a drill bit first for the hole. I then tighten both until the larger mast is basically crimped against the smaller one on all 4 sides. Again, not approved but extremely sturdy.