行政體系二元化是半總統制重要的特色之一。這個二元化的設計，將總統與總理視為二元架構中的兩個領導者，然而，當代對半總統制的研究中，對於所謂的二元體系大多定義為二元行政分權治理的情況。之所以有這樣的前提，主要原因一方面是受到法國模式的影響，另一方面是大多的半總統制國家，其現實的政治運作也凸顯了二元架構的分權治理。然而，也有一部份的半總統制國家，其二元行政的設計並非來自於權力的分配，而是基於一種將總統視為備而不用的、治理能力填補式角色的考量。這類國家，在制度設計上吻合對半總統制的定義，但具體的運作卻引起是否適合歸類於半總統制的質疑，例如奧地利、冰島、2000年之後的芬蘭、新興民主的斯洛維尼亞、甚至如歷史上的威瑪共和等。基於此，本文擬就行政體系二元化為比較的焦點，探究不同設計理念下的二元行政。本文認為，在制度上分權治理二元行政是一種分權式的二元分權結構；而具有相互補充的二元行政則是一種分時式權力置換的二元結構。這兩種類型都符合對半總統制的定義，但卻會對憲政運作造成截然不同的影響。

A dual-executive is the central characteristic of semi-presidential systems.Both the president and the prime minister are political leaders of the government.The current literature defines the relationship between the president and the prime minister as either cooperative orcompetitive. There are also many semi-presidential regimes such as the Fifth French Republic, in which the prime minister is regarded as the president’s challenger. However, there are also some semi-presidential regimes in which the president is regarded only as a political leader “in reserve”. In normal times, the government is led by the prime minister. The constitutional operation of these countries, such as Austria, Iceland, Finland after 2000, Slovenia,and Weimar Germany is similar toparliamentary systems, but with a semipresidential constitution.Therefore, dual-executive systems should be differentiated between horizontal and and vertical dual systems. Under a horizontal dual-executive system, the president and the prime minister are both political leaders at the same time. They might either share or fight for power at the head of government.However, under a vertical dual-executive system, the president is regarded as a leader only under exceptional circumstances. These two different dual-executive systems both conform to standard definitions of semi-presidentialism, but their constitutional operations are entirely different.