splitting a string with 2 different delimiters

[Skaperen]

i have a big string with 2 different delimiter characters. i want to split this big string into a list of strings for both delimiters. also, i want to keep the delimiter character at the beginning of each string it start a new string at.

with delimiters "XY", "XfooYbar" -> ["Xfoo","Ybar"]

i could make a function that does this character-by-character. but that might be too slow for the 100's of millions of characters i expect this to work with. this project needs to be limited to what comes with Python and what i can include in the code.