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I've got some fairly large dual processor server motherboards. Can i break these in two pieces so they'll fit in a smaller box?

Yes you can. I do it often. Since I mail exclusively.
The important point. You must keep the broken pieces together. That way the board can then be viewed as one complete board. I just tape or tie wrap the pieces together. I have an individual pile for each broken board. For example. If I cut or break 3 boards. I have 3 taped or tie wrapped piles.
Another point. Try not to lose components. You do want to have the complete board.
You got to get creative to a point. Judging where to break. How big to pack in box.
I never had a problem breaking boards. No negative feedback.
Here to learn like everybody else.
My evaluations are my own opinion.