I am in a relationship with two men. I can tell you about how it usually goes in our case, but have to say in advance that this obviously is only how we work
If I argue with one of them, I don't go to the other to seek comfort in that way. When I am upset I have no desire for sex, so that problem never arose for us so far. But I wouldn't go to the uninvolved party to be comforted, because those relationships are separated from each other. An argument is private stuff. Full stop. The other doesn't need to meddle in those affaires just as we never talk about sex or other personal topics outside of the respective relationship, if I am told that one of my partners doesn't wish for me to do so.
When (for example) my husband and I have a discussion and we aren't able to solve it right away, and I spend some time with my boyfriend shortly afterwards, I am affacted by this dispute. Sometimes I talk about the topic, if I need another perspective to clear my mind a bit. I never rant and I never reveal too many details. None of them would let me, btw. There has to be respect for the other relationship of the partner and it wouldn't be acceptable to take advantage of this situation.
Maybe this is due to our cohabitation. There is no complete 'outsider' in our case, that's why the affaires sometimes mingle partially. But never wholly and I wouldn't advise you to seek out a partner to solve problems with another partner of yours.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.