The two-party moderation thesis makes sense only if the main differences concern redistributive preferences in a simple model driven by an almost Marxist kind of economic determinism. The left-wing party wants to redistribute wealth and incomes more, and the right-wing party less; but both need to restrain themselves, and in appealing to the median voter, they become near-identical alternatives.
“两党意味节制”这个命题只有在特殊情况下才成立，那就是：主要的政见分歧是对（一种几乎是以马克思主义经济决定论的方式进行的）再分配的喜好程度。左翼党想要多搞点财富和收入再分配，右翼党则想少一点，但双方都必须自我节制，为了吸引中间选民，两党最终变得似异实同。