If the input file fits into memory, the easiest solution is to use str.partition():
with open("inputfile") as f:
    contents1, sentinel, contents2 = f.read().partition("Sentinel text\n")
with open("outputfile1", "w") as f:
    f.write(contents1)
with open("outputfile2", "w") as f:
    f.write(contents2)
This assumes that you know the exact text of the line separating the two parts.