A new baby is a time when the people who are going to parent the baby need extra resources from one another. The three of you chose to have babies very close together and that will mean your shared partner will be extremely stretched. It might help if all three adults and the two babies spend time together. The children are siblings anyway and having them bond should be part of the plan, regardless of how the adults feel about each other.
Game changers like marriage, illness, a new baby etc are prime times for revaluation. She might be reconsidering poly or she might be simply wanting the extra support we need from our partners and loved ones at poignant times.
Your partner has to take more onus for creating two children close together. The women both chose to have a baby, he chose to have two. One way he could do that is to make sure each mother has a days worth of supplies before he leaves them.
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