How do epinephrine and norepinephrine act together to increase cardiac output?

Answer
Epinephrine and Norepinephrine are compounds grouped under Adrenergic system. These are the neurotransmitter for the sympathetic system. These act by binding on the alpha and beta Adrenergic receptors present on the membrane of the Effector cells. These bind to the receptor and produces their effect. In heart, beta 1 Adrenergic receptors are present, when these are stimulated by epinephrine, they increase the heart rate by increasing the discharge of SA Node and also increases the contractility of the heart muscle. Also alpha receptors are present on the blood vessels. Stimulation of the these receptors of blood vessels result in vasoconstriction. This vasoconstriction increases peripheral resistance to blood flow
Cardiac output = heart rate * stroke volume

Epinephrine and norepinephrine are both stress hormones that are produced in the adrenal medulla. When epinephrine and norepinephrine act together, they cause a constriction of the blood vessels leading to the nephrons of the kidneys. This effectively halts the function of the kidneys during acute stress response that triggers flight/fight/freeze response. While this is occurring, blood pressure is increased, which in turn increases the cardiac output. The heart rate is also increased as both these hormones are cardio accelerators. This sudden heart rate acceleration is called tachycardia. As these hormones are released together in times of acute stress, there is often both an increased heart rate response as well as an increase in blood pressure through the sympathetic nervous system reaction.