Join 6"x6" wooden beams horizontally, bridge building

[svenork]

I'll start off by saying I have no background in engineering.

My wife and I are interested in building a bridge across a 20-25 foot wide creek. Due to ordinances, we cannot put anything in the stream (such as support posts), so it will only be supported at the two ends. Local timber supply shops have max length of 16 feet for 6x6 pressure treated wood, so my initial thought was to join two 6x6 beams together. I've located some pre-drilled 304 steel plates that are 12"x4" and 5mm thick. I was thinking that if I attached four of them on the four surfaces around the joint that it would be sturdy enough to support weight. Or perhaps a better solution would be to place another 8-foot section of 6x6 post directly under the joint (4 feet overlapping one beam and 4 feet overlapping the other). Am I completely off-base? Does anyone have alternative suggestions for the end-goal? We have a modest budget and would prefer to build it ourselves, but not sure if we're out of our depth. It conceptually seems simple enough - as long as we can create a sturdy "beam" across the gap, we can construct as many of those as necessary and then just apply the decking/rails/etc as-needed.

Thanks for any help!

[Cragyon]

"We have a modest budget and would prefer to build it ourselves"

Appreciated, however without loading specified, bank support known, sil / ground characteristics understood etc., etc. you clearly are out of your knowledge and skill sets.

Get a local engineer to do the design..