Quite literally, I imagine it would go something like:
Do you take this woman to be your wife? I do.
Do you take this woman to be your husband's second wife? I do.
Do you take this man and woman to be your husband and second wife? I do.
That's how three adults could all consent.
You can argue that someone is being forced to say yes, but you could argue that in any type of marriage or contract.
That said all of the legal agreements become far more complicated and likely null and void if one person drops out or is added. I think an argument could be made, but with the complication you're basically saying polymarriages would be like corporations, with their associated complexity.
Polygamy as practiced now is a "bring more childbearing women" into the flock type thing, which is by nature non-equitable. I doubt most folks in a polygamous marriage (by choice) would want to get involved in a polyamorous marriage and as such, I think you can't legally support a non-equal union of that sort.
Hmmm, please excuse my ignorance in such matters. I only married one woman and that is quite enough.
>you're basically saying polymarriages would be like corporations
Yes, exactly. Well put.
>with their associated complexity
I can't even begin to imagine the complexity, legal or relational. However, I can imagine situations where such an arrangement could be beneficial to the individuals involved.