Well, you could purchase a male (that is what I did with both my M. bipartus and M. choati because I was trying to get a primary male partner) but if you choose to "convert" a female, buy one that is smaller (which would be ideal). However, I suspect that any two females will "work it out" as long as they are both relatively smallish. The males are considerably larger and primary males have slightly more intense coloration.
"Like most, if not all tropical wrasses Macropharyngodon is a protogynous hermaphrodite. That is, most begin life as females and males develop from females as needed. To complicate matters there are primary males (those born as males) whose coloration is more intense and whose testis are cylindrical and white with a seminal duct. Generally, there is one dominant male, usually primary, to a number of females. However, if that primary male is eliminated the highest ranking female transforms, often in a matter of hours, into a fully functional male. These are called secondary males, and though fully functional and able to maintain control over the harem, their coloration is not as intense as that of primary males and their testis “reflect their ovarian origins." (Baensch, Marine Atlas)"