Bism Illah wa as salaamu alaykum wa Rahmat Allah wa Barakatuh.
Let me first say that I haven't met my co-wife yet. When that happens, inshaa'Allah I will post about it. However, I want to address why I'm grateful to her. Her specifically? No. To any co-wife my husband chooses, as long as she is pleasing to Allah.
There many things a husband and wife learn about each other in a marriage. Some of those things, once learned, change how we view our spouse. Some may cause us to avoid certain situations or scenarios. Some may even give us an aversion to something. Although they can be forgiven, mistakes, weaknesses, or serious actions disliked by Allah cannot be erased. A glass, once broken, cannot be put back together perfectly.
Over the years, some glasses have been broken in our relationship. Correct me if I'm wrong, but I think this is normal in any relationship. It may depend on personalities and the nature of the issues involved as to whether it has much affect on a marriage or not. Is it a bad thing for glass to be broken? No, it is Allah's Qadr and comes to bring something to light that needs to be taken into account, or to change something in the relationship. Sometimes Allah reveals things to us, for our particular situation, so we are forewarned and can act accordingly. Knowing certain things about my husband enables me to approach him in ways specific to his situation and steer clear of certain situations that I know will be a fitnah.
My co-wife doesn't have any of that history. She may, inshaa'Allah, never know the things I know about our husband. She may, inshaa'Allah, never have the experiences I've had. No glasses are broken yet. Of course, some glasses will break...that is the nature of relationships. However, they may not be the same glasses that broke with me. With a fresh outlook, she will not have the inclination to avoid certain things or any aversions. She will give her all, innocently, without bad memories or previous knowledge to cause her to hold back. She will fill in the gaps, those I simply cannot fill. She will, inshaa'Allah, welcome the side of my husband I don't welcome. She will give him the things that are missing from me.
Now, I have a favorite supermarket (Wholefoods) and I prefer to shop there most of the time, but there are always some things I need that I cannot get from there. So I also go to Trader Joes to get the balance of the things I need. That's how I see all of this. Having a co-wife is pure benefit for me...because while he is happy with her and appreciative of what she gives him, he is also happy with me. Any resentment or disappointment he may have harbored for me because of me not fulfilling his every want, or not providing an environment for him to freely express certain sides of his personality, will be gone because he has those needs fulfilled now.
Because of my co-wife, my husband is happier. Because of her, my family has more structure in our daily schedule and his allocation of time. Because of her, my husband is nicer, more loving, and more appreciative of me. Because of her, he is more generous with us. He has a chance to miss the children and I. He has a chance to look forward to those things he loves in our home...and to see me in a whole new light. Because of her, the weight of raising two step-children is going to be lightened inshaa'Allah. And because of her, we will have more time to focus on what's important and making the most of our time, when my husband is home and when he is away.
In honesty, it isn't important for us to be great friends...it is just important that our husband reap the benefits that Allah has placed in her for him.