what i do not understand is what are you getting out of this? I mean that's a totally symbolic act but would he accept the same for you or it is one sided?
If you wish to be in a polygamy or polygyny relationship with other adults then that is your right. I would still advise caution if you have not met the other wife as those who generally practice this lifestyle have the agreement of all partners on someone else being brought into the marriage. If you have not personally met this other wife, I would suggest that you do so before you even consider taking the relationship further. It has to be cohesiveness between all to make this type of lifestyle work. Make sure it is there for you. If for some reason that you do not spend time with the other wife and husband it is time to walk away. Something then is not transparent with the offer.