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):)"
res = [v.rstrip() for v in re.split(pattern, s, 0, re.M) if v]

print(res)