Important to note: you really, seriously, don't need the extra strength the bottom piece gives. It is simply a complete and utter waste of oldgrowth forrests to strive for that quality. Loadbearing posts and beams are made in quality assured, tested wood.

Often times, like 99% of the time, the dimensions of the wood isn't determined by the strength required, but rather the insulation thickness required, size of internal installations like HVAC, or soundproofing requirements.