Reading man flock(2):
EWOULDBLOCK The file is locked and the LOCK_NB flag was selected.
So getting EWOULDBLOCK means the file is already locked. If it is guaranteed that your two processes are the only ones involved, they will never get EWOULDBLOCK on the same file at the same time.
Please note that threads is a different story. Threads normally share file descriptors, so several threads within the same process can call flock() successfully on the same file.