Why does the shulchan aruch specify that it is okay if the mother only converted and not if both parents converted. If both parents are converts would that not make it even better? Or if it is both parents is there a fear one converted for the other and therefore you cannot tell if they truly converted of their own volition? Furthermore, what if both parents converted prior to meeting one another?

The law stipulated in the Shulchan Aruch is based upon the interpretation the Talmud(Kiddushin 78) and codified in the Rambam (הלכות אסו"ב יט:יב) that the marriage restriction of a cohen to a convert is only in a case where both parents are converts. As known ,the Torah gave special laws to whom a cohen may marry in order to preserve his special status. There is an opinion in the Talmud which holds that a cohen may not even marry the child of a union in which even one parent is a convert. However, that opinion was not accepted. The accepted opinion is that if one parent is born Jewish and the spouse is a convert, than a cohen may marry the child born of that union.