Lol jess when I was a wee lass of 13 the only thing I used to write in my diary was the days I had sex, because back then that was all that mattered - getting laid.
Although, i've never had sex with an ex or accidentally forgot to use protection...
To the original poster, because you had sex with 2 different guys so close together, the only way to tell 100% is by a paternity test, unless as jess suggested they are different race or something.
Although there is a higher chance of your ex being the sperm donor due to the fact you didn't use a condom/birth control - it doesn't matter that he didn't ejaculate.