Of all the unique things that you COULD do on or to a bike this is one where you should just lay down until the feeling goes away.

Every issue you pointed out is far more easily dealt with by using good materials and spending time on the details of the setup. As noted with decent to upper range brakes they are all excellent stoppers that will easily lock the rear or send you over the bars. If they feel spongy and you don't like that then buy the premium compresionless brake housing and die formed pre-stretched cables so you eliminate any sponginess in the system. A two caliper solution off one lever won't reduce sponginess at all. In fact with the inevitable differences in the two cable systems off the one lever you'll generate MORE sponginess, not less.