import re

s = "person alpha:\nHow are you today?\n\nperson beta:\nI'm fine, thank you.\n\nperson alpha:\nWhat's up?\n\nperson beta:\nNot much, just hanging around."
pattern = r"^person (?:alpha|beta):\n(?:(?!person (?:alpha|beta):).+(?=\n|$))*"
print(re.findall(pattern, s, re.M))