From the way Rambam Ishut 3:9 is written, she is completely mekudash to the man. The half just means that he is using language as if he had married two women (which is allowed). Thus, they are married completely and there would be no nafka minah in the matter.
The Rambam (as I read it) says that once the kiddushin "takes hold" it is considered a full kiddushin and must follow all the appropriate halachos as it says in the pasuk. I understand the analogy to be as if he had been married to two wives and one of them had died or been divorced. The remaining wife is in the same situation as the wife married by half.
If he takes another [wife] for himself, he shall not diminish her
sustenance, her clothing, or her marital relations.
Note that if he had married two women, he would be required to share his time between them and provide for them both. Mezonos means the living expenses of the household, however many women are involved. A man married to two women does not spend twice as much on living expenses. That is, a man married to two women, who then divorces one of them, does not still only spend half his time with the wife that remains. This case, would be considered as similar to that situation. As a result, once she is completely mekudash to him, she is treated in every respect as his full wife.
Rambam Hilchos Ishut 10:4 shows what is done if he is married to two wives as far as conjugal rights. Halacha 3 shows that he must fully fund the household of each of his wives in any case. Thus, marrying half would not decrease the mezonos that he is responsible for.
We see here
This mitzvah applies in all times and all places, but only to men. It
is discussed in the Talmud in the fourth and fifth chapters of
Kesubos. In the Shulchan Aruch, it is found in Orach Chaim 240. It is
number 262 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and
number42 of the 194 negative mitzvos that can be fulfilled today in the
Chofetz Chaim’s Sefer HaMitzvos HaKatzar.
The second half of the halacha shows under what circumstance the kiddushin does not take hold at all.
When a man tells a woman, "Become consecrated to my half," she is
consecrated. To what can this be compared? To his saying, "May you
become my wife, and also another woman." And so, she has only half a
If, however, he said: "Half of you is consecrated to me," she is not
consecrated. For a woman cannot be consecrated to two men. Similarly,
if he says, "Behold, you are consecrated to me and to him," the woman
is not consecrated.