I find the idea of poly interesting, but one I have personally rejected for a variety of reasons.
To answer the OPs original question, practical concerns I would have are...
Financial - have you discussed spending money on the secondary relationship vs the primary, eg taking the secondary partner out for dinner vs saving for the children's future. Might be something all 4 of you need to discuss as secondary woman's DH is financially involved in her life too. Unless both couples have similar incomes and disposable cash this could easily be seen as unfair.
Splitting up - have you discussed what happens I the event of either primary relationship failing? Or (heaven forbid) someone dying? If this is a very long term thing what happens in retirement etc? Is there ever a stage when you might all live together or retire into the same sheltered accommodation etc? Should you make any specialist provision in wills etc?
Telling the children - do you ever want to do this, why or why not. What happens if older children suspect an affair (misunderstanding the consensual element of poly) or if others outside the relationship reveal it? Do the other couple have children and do they / might they know each other eg same school, same clubs?
STDs - what happens if someone tests positive for an STD? Eg other woman's husbands partner. Under what circumstances would sex be halted or stopped permanently?
Other major life changes - who has what veto over major life changes like job or career change, moving house locally or further away, taking further partners into the situation. or if someone gets seriously ill. Who has next of kin status, who wants a say in end of life medical situations, who gets to visit the hospital if someone has an accident? who is involved in decisions about conceiving future children - if you wanted a fifth child that might have implications for your libido and therefore your husbands sexual desire for the other woman, so does she get a say?
If there is infidelity who expects to tell whom? Eg if OP had an unsanctioned affair or one night stand, is the secondary relationship expecting to be informed?
Basically, think about the reasons any relationship struggles - money, unfaithfulness, unfair burden of domestic work, perceptions of freedom and value (eg unthanked domestic drudge vs celebrated career), sickness, old age and/or dementia, and work out what that means for your setup.
One of the reasons I think monogamy works for most is that we have a huge cultural history of how to deal with these situations in a monogamous marriage. Next of kin rules, inheritance, spousal pensions etc are all set up with this in mind and most people have clear expectations of these situations as well as the sexual fidelity aspect of marriage. If that set up doesn't work for you he you need to discuss everything, from scratch, and make your own agreement. It sounds like you are sensible enough to be doing that.
Good luck, be safe and enjoy it!