actually you mean harem which is more than 2 girls. most polygamy relationships revolve around 1 guy 2 girls. Harem is an open relationship for the male, he is free to consort or date any girl he wants to hit on if she reciprocates the attention and interest back. However every girl in the harem is 100% faithful to the guy.
In Polygamy it is similar, but in reality it always usually involves just 3 people and no more than that. Sure you could create an harem through polygamy, but realistically it would never happen.
In polygamy your not setting out to date as many women as possible, and eventually have them all living together, which in almost all cases usually occurs depending on harem size of course. In polygamy you could be dating just the one girl then meet another or your fall for your girlfriend's friend which strangely is mostly the usual way a poly relationship occurs. Unlike in these adult visual novels, a girl friend even in an harem, is not going to tell you to go out and date who you want and as many as you want bleh bleh. She is faithful to you, which mostly means she doesn't exactly like what your doing, but she is powerless to stop you, or she could loose you so she comes to terms with that fact.
I am polygamous myself and in a relationship with 2 women, one is the best friend of the other, which makes things much easier and harmonious. I have never been monogamous and never will be. Monogamy generally only exists because of religion, unless your a Mormon or non religious like myself. I have a lot of experience with polygamy. No it's not 3 some heaven like some might think, neither are bisexual. I either go to my main girlfriend or the other, no big problem they both accept it, we go on holidays all together (the 3 of us), go pretty much everywhere together.