Reply to Wolfi
Wolfi, I don't know if you've started your project, since I only read your post today 5/7.
However, I wonder why you are struggling to unite two 8 foot posts to create one 4" x 4" x 16 feet high? You can buy 16 foot posts, as noted in the following site example.
WIDE ASSORTMENT OF SIZES AND LENGTHS
Cedartone Classic pressure treated wood is available in a wide assortment of popular sizes and lengths of lumber for any outdoor project.
DIMENSIONS: PIECES PER LIFT:
2x4xR/L-8' through 16'
2x6xR/L-8' through 16'
2x8xR/L-8' through 16'
2x10xR/L-8' through 16'
4x4xR/L-7' through 16' (except 9')
1x6x5' & 6' S4S
1x6xR/L-8' through 16' S4S