A good polygamist relationship has to have peace between the sister wives. Otherwise, you just have chaos.
This chaos hurts the man the most, he's the one who has to hear about it.

Ultimately when your wives can't get along with each other, YOU are the one who suffers the most.

I guess one way to insure peace is to not show favoritism. Now, let's be honest... It's okay to have a favorite wife.
However, having favoritism is different than showing favoritism. It's like having children, yea... one is gonna be better than the other, but you don't make it obvious.
However, you don't treat them totally equal... I mean, if one wife is better behaved than her sister wife, then it's unfair to treat them both the same if one is the trouble maker and the other isn't.
You should reward the wives that don't get jealous easily.

It's also not a good idea to just marry two completely different women who don't really like each other... It's best to find women who have some sort of history together.. like a chinese laotong sisterhood or something. They're not as likely to fight if they're close to one another.

Now, unless all your girls are bi, it's a good idea to keep the number under 3... Cause your dick has it's limits. Men don't get multiple orgasms like women do... Now, if they are comfortable satisfying each other, you could have lots of wives. You could have seven wives and sleep with a different one each day of the week. If they are fine doing it with one another, that can mitigate some of your sexual responsibility.
But you have to alternate these girls weekly.
If one girl is always monday and the other is saturday. The monday girl is gonna think you see less of her.
So it's best not to assign your wives a specific day of the week permanently.

One problem with having a harem too big, is that they start to prefer the company of each other over you... And that can be hurtful to the mans self-esteem to know that his wives prefer each other over him. This could cause an issue bigger than them fighting one another... and that's them ganging up on you.
There's nothing worse than several women being mad at you all at the same time

But overall it's just important to remember that your happiness depends on their happiness.
And if they're not happy, you're gonna get nagged in stereo.