This seems odd to me since this would tend to weaken the beam? Does
anyone really do this? ALso I would thin kPT Plywood would be a lot
less durable than 2x10 PT? Is it really practical?


Also, let's say my header is 12' long. What carriage bolt spacing
should I use? I was thinking 24", staggered on-on-top, one-on-bottom?


Which is it a header or a beam? A header spans the space over a door or
window. Which I suppose is a type of beam. Generally you need 1" of header
for every foot of span. Therefore a a 12' header should be constructed of
2x12 lumber, not the 2x10 you suggested.

For beams.... all bets are off. it depends on what you are supporting.

Please do not take offense, but the fact that your question is poorly formed
suggests that you are a bit in over your head. You should speak to a pro
(Engineer or experienced framer)

-Steve