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Yes providing you can join them at multiple points up the posts. All bolt would be fine. If you have a welder, I would weld two together with short seam welds at several points along the length. Probably not necessary to weld the whole length. Do the same to the other two. Then bolt the two resulting sections together with 1/4" bolts at intervals along the lengths.
Whether it's stronger or weaker would depend on the wall thickness of your 50mm posts compared to the wall thickness of a 100mm post. A 100mm RHS would probably have thicker wall. It's a moot point. The construction should be more than strong enough to hold a moderately large mount.
Joe
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