Wouldn't it reduce their reproductive opportunities? Since a female can only be impregnated by one male/halfsexed then Harems would naturally decrease their reproductive opportunities. If a half sex was a consort to a boreal couple then there would be only a 50% chance that any child born of the female would be theirs and that's assuming they would be allowed to finish in the females vagina. Larger harems would only decrease the chance of their seed bearing fruit.
Heteropaternal superfecundation - where a single pregnancy has multiple children with different sires - is a thing in real-life, and has been documented to happen among even supposedly monogamous couples. I'd imagine it would be a lot more common among societies where females having multiple sex partners is common, such as ausar and boreal elves (at least, the ones with consorts and/or additional spouses). Even if we discount that possibility, your logic would only apply on a per-pregnancy basis and things would, on average, even out over the course of multiple pregnancies.
Then again, when I said what I said I must have assumed the consort-taker was always female. If the taker was male (including another half-sexed), and assuming no cheating, the only viable reproductive partners for the half-sexed would be any females who are also lovers with the male, and that would still likely require permission from the male him/herself. In that situation, I could understand reduced breeding opportunities. For female consort-takers, I still think they'd get increased breeding opportunities since they have guaranteed access to the taker herself, plus (again) any of her other lovers she lets the half-sexed breed with.
Half-sexed who aren't consorts or additional spouses would have to find a lover of their own, but unless females are significantly less likely to be single than non-half-sexed males, I don't see that having much of an impact on their access to breeding opportunities. And, of course, if a half-sexed is the head of the household it's entirely possible for him/her to have consorts him/herself, and he/she is practically guaranteed to have kids.