In answer to your question Pixiagi - I wouldn't be able to have both of them at the same time (and if I was still with my ex I'd leave him in a heart beat - there was no love on his side of the equation) I would have to chose. I've never given it much thought actually. But now that I have let me tell you what I think (and as always this is my opinion). In this world the way it is today, it's always the man who gets to have the extras - the extra bit on his paycheck, the extra serving at dinner time, the extra wife... it comes from being in a patriarchal society. Do I believe humans are a monogamous species? No way... but we have the intelligence to chose how we want to live, and if polygamy is what we chose to do go for it. I'm just not wired that way. Call me selfish, call me an idiot, call me whatever you want - I just can't do it.
Now if I was raised that way, then yeah I'm sure I wouldn't have a problem with the whole thing, but I was raised as one man one woman.
If all adults are consenting (and adult is a big word here - there have been cases where a man has taken multiple wives and some of them have been a hell of a lot younger than they should be), and they all provide for the family unit (including each other's kids and what not) then I see nothing wrong with it - for them.
And I have been in this situation - kinda. My ex brought in a woman right after I had my oldest daughter to "help out with the chores and the kid"... what he really wanted her for was an on call on site sex partner until I could have sex again. I have to stop and wonder if the people who do have multiple spouses are really in love with any of them or if they just want to romp with a different person every night or have an on call orgy without having to deal with the whole adultery thing.