How are systolic and diastolic similar?

Blood Pressure. Blood pressure is related to the ability of the heart to pump blood and to the stiffness of the arteries. The systolic blood pressure or "top number" is dependent on the hearts ablility to pump blood into the arteries. The diastolic pressure or "bottom number" relates to the pressure in the arteries as the heart relaxes. Both will increase as the artery wall becomes stiffer with age or plaque.
Sys/dias. Both systolic and diastolic are definitions and in that is their basic similarity. The other answers describe the physiologic similarity.