You might be able to split two posts lengthwise and split half of three posts, then splice these together with lots of wood glue and coarse threaded rod that you run through the posts. Then you should be able to assemble 2 posts at twelve meters. The question is whether the joint will be strong enough. It's in the middle of the bridge where the load is the greatest.
Another possibility might be to buy 4 pieces of 4-meter sturdy steel pipes that you use to splice the posts together.
A suspension bridge is probably the cheapest to build. Then you can use the posts to build pylons.
Another possibility might be to buy 4 pieces of 4-meter sturdy steel pipes that you use to splice the posts together.
A suspension bridge is probably the cheapest to build. Then you can use the posts to build pylons.