It seems like a couple steel bolts would be stronger than the aluminum they replaced.
Seems like 2 piece calipers would really just be a cost saving measure.
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Many of the lighter brakes use a one-piece caliper. It's probably quicker to machine halves than to set up and do multiple machining operations on a single part. But you can use lighter bolts to hold two-piece calipers together if you wish.
'05 Giant NRS C1
'06 Giant TCR Advanced
'68 Schwinn Typhoon
I believe monoblock calipers are stiffer per gram than a 2-piece caliper.
I guess there's also not that joint for it to spread apart or flex at.
More material in the bridge or a thicker bridge will lend to a stiffer caliper.