A 1-and-2-half stage could have gentler acceleration curves than Atlas. That launcher, derived from an ICBM design, accelerated steeply to about 7g, dropped the booster section, then accelerated back to nearly 8g on the sustainer.
With two separate booster sections and a smaller sustainer, it could reduce its thrust in smaller steps, keeping the g-force peaks lower and the troughs higher, maintaining similar average acceleration while putting less stress on the payload.
The big disadvantage, of course, is that a booster section that can safely detach from a rocket in flight is heavier than the same engines would be if they were permanently attached. I note that Atlas’ booster masses about twice as much as a pair of H-1 engines while producing comparable total thrust.