If that isn't a description of a shared power, I don't know what would be.
Article 1, Section 3: The Congress shall have power ... To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water; To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years; To provide and maintain a Navy; To make Rules for the Government and Regulation of the land and naval Force
Article 2, Section 1: The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States
This again shows shared power. Neither branch can make war if the other refuses to proceed. The President cannot make war unless Congress provides the funds, and Congress cannot order the military to make one move except through the President.
If these aren't shared powers, what would you call them?
See? That wasn't so hard, was it?
Buchanan should have been more careful with his verbiage, as his blanket statement is misleading.
'Congress has the power to declare war and consent to peace' would have been a lot more accurate, IMHO.