The idea is to spend two actions to focus on them so we don't have to pick one or the other.
One action is dedicated to decreasing the cost, the other action is dedicated to improving the design. I imagine it's akin to assigning two research teams to the reactor, with one team focusing on one design goal and the other team focusing on the other design goal; they'll meet up to share notes and ensure they're on the same page, but with two teams working the problem, each will be able to focus on fixing their particular goal, negating the 'trying to fix too many things at once' problem.
Or, alternatively, we get one die roll for cost, another die roll for design improvement.
I don't see how it's any different than if we spent one week assigning an action to cost and then the next week assigned an action to improving the design, except we're moving the action from the second week to a second spot in the first week instead.